Dennis L Lichtenberger

Interests

Teaching

Inorganic Chemistry,Organometallic Chemistry,Computational Chemistry,Undergraduate Independent Studies

Research

Catalysis, Electrocatalysis, Solar Fuels, Molecular Electronics, andMultiple Metal-metal Bonds studied by Photoelectron Spectroscopy,Electrochemistry, Photochemistry, Synthesis, and Computations

Courses

Scholarly Contributions

Journals/Publications

• Karayilan, M., McCleary-Petersen, K. C., Hamilton, M. O., Fu, L., Matyjaszewski, K., Glass, R. S., Lichtenberger, D. L., & Pyun, J. (2020). Synthesis of Metallopolymers via Atom Transfer Radical Polymerization from a [2Fe-2S] Metalloinitiator: Molecular Weight Effects on Electrocatalytic Hydrogen Production. Macromolecular rapid communications, 41(1), e1900424.
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  Small molecule biomimetics inspired by the active site of the [FeFe]-hydrogenase enzymes have shown promising electrocatalytic activity for hydrogen (H ) generation. However, most of the active-site mimics based on [2Fe-2S] clusters are not water-soluble which limits the use of these electrocatalysts to organic media. Polymer-supported [2Fe-2S] systems, in particular, single-site metallopolymer catalysts, have shown drastic improvements for electrocatalytic H generation in aqueous milieu. [2Fe-2S] complexes functionalized within well-defined macromolecular supports via covalent bonding have demonstrated water solubility, enhanced site-isolation, and improved chemical stability during catalysis. In this report, the synthesis of a new propanedithiolate (pdt)-[2Fe-2S] complex bearing a single α-bromoester moiety for use in atom transfer radical polymerization (ATRP) is demonstrated as a novel metalloinitiator to prepare water-soluble poly(2-dimethylaminoethyl methacrylate) grafted (PDMAEMA-g-[2Fe-2S]) metallopolymers. Using this approach, metallopolymers with controllable molecular weights (M = 5-40 kg mol ) and low dispersity (Đ, M /M = 1.09-1.36) are prepared, which allows for the first time observation of the effect of the metallopolymers' chain length on the electrocatalytic activity. The ability to control the composition and molecular weight of these metallopolymers enables macromolecular engineering via ATRP of these materials to determine optimal structural features of metallopolymer catalysts for H production.
• Kleine, T. S., Glass, R. S., Lichtenberger, D. L., Mackay, M. E., Char, K., Norwood, R. A., & Pyun, J. (2020). 100th Anniversary of Macromolecular Science Viewpoint: High Refractive Index Polymers from Elemental Sulfur for Infrared Thermal Imaging and Optics. ACS Macro Letters, 9(2), 245-259.
• Glass, R. S., Pyun, J., Lichtenberger, D. L., Brezinski, W. P., Karayilan, M., Clary, K. E., Pavlopoulos, N. G., & Evans, D. H. (2019). Water-soluble and air-stable [2Fe-2S]-metallopolymers: A new class of electrocatalysts for H2 production via water splitting. Phosphorus, Sulfur, and Silicon and the Related Elements, 194(7), 701-706.
• Humphries, M. E., Wusterbarth, E. S., & Lichtenberger, D. L. (2019). Weak Acids with Super-Electron-Donor Dimetal Complexes: Synergy in Bifunctional Activity. Polyhedron, 158, 471-477. doi:10.1016/j.poly.2018.11.001
• Karayilan, M., Brezinski, W. P., Clary, K. E., Lichtenberger, D. L., Glass, R. S., & Pyun, J. (2019). Catalytic Metallopolymers from [2Fe-2S] Clusters: Artificial Metalloenzymes for Hydrogen Production. Angewandte Chemie (International ed. in English), 58(23), 7537-7550.
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  Reviewed herein is the development of novel polymer-supported [2Fe-2S] catalyst systems for electrocatalytic and photocatalytic hydrogen evolution reactions. [FeFe] hydrogenases are the best known naturally occurring metalloenzymes for hydrogen generation, and small-molecule, [2Fe-2S]-containing mimetics of the active site (H-cluster) of these metalloenzymes have been synthesized for years. These small [2Fe-2S] complexes have not yet reached the same capacity as that of enzymes for hydrogen production. Recently, modern polymer chemistry has been utilized to construct an outer coordination sphere around the [2Fe-2S] clusters to provide site isolation, water solubility, and improved catalytic activity. In this review, the various macromolecular motifs and the catalytic properties of these polymer-supported [2Fe-2S] materials are surveyed. The most recent catalysts that incorporate a single [2Fe-2S] complex, termed single-site [2Fe-2S] metallopolymers, exhibit superior activity for H production.
• Kleine, T. S., Lee, T., Carothers, K. J., Hamilton, M. O., Anderson, L. E., Ruiz Diaz, L., Lyons, N. P., Coasey, K. R., Parker, W. O., Borghi, L., Mackay, M. E., Char, K., Glass, R. S., Lichtenberger, D. L., Norwood, R. A., & Pyun, J. (2019). Infrared Fingerprint Engineering: A Molecular-Design Approach to Long-Wave Infrared Transparency with Polymeric Materials. Angewandte Chemie (International ed. in English), 58(49), 17656-17660.
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  Optical technologies in the long-wave infrared (LWIR) spectrum (7-14 μm) offer important advantages for high-resolution thermal imaging in near or complete darkness. The use of polymeric transmissive materials for IR imaging offers numerous cost and processing advantages but suffers from inferior optical properties in the LWIR spectrum. A major challenge in the design of LWIR-transparent organic materials is that nearly all organic molecules absorb in this spectral window which lies within the so-called IR-fingerprint region. We report on a new molecular-design approach to prepare high refractive index polymers with enhanced LWIR transparency. Computational methods were used to accelerate the design of novel molecules and polymers. Using this approach, we have prepared chalcogenide hybrid inorganic/organic polymers (CHIPs) with enhanced LWIR transparency and thermomechanical properties via inverse vulcanization of elemental sulfur with new organic co-monomers.
• Brezinski, W. P., Karayilan, M., Clary, K. E., McCleary-Petersen, K. C., Fu, L., Matyjaszewski, K., Evans, D. H., Lichtenberger, D. L., Glass, R. S., & Pyun, J. (2018). Macromolecular Engineering of the Outer Coordination Sphere of [2Fe-2S] Metallopolymers to Enhance Catalytic Activity for H2 Production. ACS Macro Letters, 7(11), 1383-1387.
• Brezinski, W. P., Karayilan, M., Clary, K. E., Pavlopoulos, N. G., Li, S., Fu, L., Matyjaszewski, K., Evans, D. H., Glass, R. S., Lichtenberger, D. L., & Pyun, J. (2018). [FeFe]-Hydrogenase Mimetic Metallopolymers with Enhanced Catalytic Activity for Hydrogen Production in Water. ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 57(37), 11898-11902.
• Stein, B. W., Yang, J., Mtei, R., Wiebelhaus, N. J., Kersi, D. K., LePluart, J., Lichtenberger, D. L., Enemark, J. H., & Kirk, M. L. (2018). Vibrational Control of Covalency Effects Related to the Active Sites of Molybdenum Enzymes. Journal of the American Chemical Society, 140(44), 14777-14788.
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  A multitechnique spectroscopic and theoretical study of the CpM(benzenedithiolato) (M = Ti, V, Mo; Cp = η-CH) series provides deep insight into dithiolene electronic structure contributions to electron transfer reactivity and reduction potential modulation in pyranopterin molybdenum enzymes. This work explains the magnitude of the dithiolene folding distortion and the concomitant changes in metal-ligand covalency that are sensitive to electronic structure changes as a function of d-electron occupancy in the redox orbital. It is shown that the large fold angle differences correlate with covalency, and the fold angle distortion is due to a pseudo-Jahn-Teller (PJT) effect. The PJT effect in these and related transition metal dithiolene systems arises from the small energy differences between metal and sulfur valence molecular orbitals, which uniquely poise these systems for dramatic geometric and electronic structure changes as the oxidation state changes. Herein, we have used a combination of resonance Raman, magnetic circular dichroism, electron paramagnetic resonance, and UV photoelectron spectroscopies to explore the electronic states involved in the vibronic coupling mechanism. Comparison between the UV photoelectron spectroscopy (UPS) of the d M = Mo complex and the resonance Raman spectra of the d M = V complex reveals the power of this combined spectroscopic approach. Here, we observe that the UPS spectrum of CpMo(bdt) contains an intriguing vibronic progession that is dominated by a "missing-mode" that is composed of PJT-active distortions. We discuss the relationship of the PJT distortions to facile electron transfer in molybdenum enzymes.
• Abdul-Futouh, H., Almazahreh, L. R., Sakamoto, T., Stessman, N., Lichtenberger, D. L., Glass, R. S., El-khateeb, M., & Weigand, W. (2017). [FeFe]​-​Hydrogenase H-​Cluster Mimics with Unique Planar μ-​(SCH2)​2ER2 Linkers (E = Ge and Sn). Chemistry, A European Journal, 23(2), 346-359.
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  Analogs of the [2Fe-​2S] subcluster of hydrogenase enzymes in which the central group of the three-​atom chain linker between the sulfur atoms is replaced by GeR2 and SnR2 groups are studied. The six-​membered FeSCECS rings in these complexes (E = Ge or Sn) adopt an unusual conformation with nearly co-​planar SCECS atoms perpendicular to the Fe-​Fe core. Computational modeling traces this result to the steric interaction of the Me groups with the axial carbonyls of the Fe2(CO)​6 cluster and low torsional strain for GeMe2 and SnMe2 moieties owing to the long C-​Ge and C-​Sn bonds. Gas-​phase photoelectron spectroscopy of these complexes shows a shift of ionization potentials to lower energies with substantial sulfur orbital character and, as supported by the computations, an increase in sulfur character in the predominantly metal-​metal bonding HOMO. Cyclic voltammetry reveals that the complexes follow an ECE-​type redn. mechanism (E = electron transfer and C = chem. process) in the absence of acid and catalysis of proton redn. in the presence of acid. Two cyclic tetranuclear complexes featuring the sulfur atoms of two Fe2S2(CO)​6 cores bridged by CH2SnR2CH2, R = Me, Ph, linkers were also obtained and characterized.
• Gladysz, J. A., Bochmann, M., Fogg, D. E., Gabbai, F. P., Lichtenberger, D. L., Liebeskind, L. S., & Mindiola, D. J. (2014). The 2014 Organometallics Symposium.. Organometallics, 33(19), 5049-5051.
• Hall, G. B., Kottani, R., A., G., Yamamoto, T., Evans, D. H., Glass, R. S., & Lichtenberger, D. L. (2014). Intramolecular electron transfer in bipyridinium disulfides. Journal of the American Chemical Society, 136(10), 4012-4018.
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  Abstract: Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4′-bipyridyl-3,3′-disulfide (1) and the di-N-methylated derivative (22+) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S-S bond to form the dithiolate. In contrast, compound 22+ is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S-S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S-S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials. © 2014 American Chemical Society.
• Harb, M. K., Daraosheh, A., Goerls, H., Smith, E. R., Meyer, G. J., Swenson, M. T., Sakamoto, T., Glass, R. S., Lichtenberger, D. L., Evans, D. H., El-khateeb, M., & Weigand, W. (2014). Effects of Alkane Linker Length and Chalcogen Character in [FeFe]-Hydrogenase Inspired Compounds.. Heteroatom Chemistry, 25(6), 592-606.
• Ignacio, J., Juárez-Saavedra, P., Paz-Michel, B., Leyva-Ramirez, M. A., Rajapakshe, A., Vannucci, A. K., Lichtenberger, D. L., & Paz-Sandoval, M. A. (2014). Phosphine-substituted (η5-pentadienyl) manganese carbonyl complexes: Geometric structures, electronic structures, and energetic properties of the associative substitution mechanism, including isolation of the slipped η³-pentadienyl associative intermediate. Organometallics, 33(1), 278-288.
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  Abstract: The molecule (η5-Me2Pdl)Mn(CO)3 (η5-Me2Pdl = 2,4-dimethyl-η5- pentadienyl) has been prepared by a new method and used as a starting material to prepare the molecules (η5-Me2Pdl)Mn(CO) n(PMe3)3-n (n = 2, 1) by phosphine substitution for carbonyls. The first carbonyl substitution is achieved thermally in refluxing cyclohexane, and the second carbonyl substitution requires photolysis. At room temperature in benzene the associative intermediate (η3-Me2Pdl)Mn(CO)3(PMe3) that precedes the initial loss of carbonyl is observed. Single-crystal structures are reported for all complexes, including the associative intermediate of the first substitution in which the pentadienyl ligand has slipped to the η3 bonding mode. These molecules offer an opportunity to examine fundamental principles of the interactions between metals and pentadienyl ligands in comparison to the well-developed chemistry of metal cyclopentadienyl (Cp) complexes as a function of electron richness at the metal center. Photoelectron spectra of these molecules show that the Me2Pdl ligand has π ionizations at energy lower than that for the analogous Cp ligand and donates more strongly to the metal than the Cp ligand, making the metal more electron rich. Phosphine substitutions for carbonyls further increase the electron richness at the metal center. Density functional calculations provide further insight into the electronic structures and bonding of the molecules, revealing the energetics and role of the pentadienyl slip from η5 to η3 bonding in the early stages of the associative substitution mechanism. Computational comparison with dissociative ligand substitution mechanisms reveals the roles of dispersion interaction energies and the entropic free energies in the ligand substitution reactions. An alternative scheme for evaluating the computational translational and rotational entropy of a dissociative mechanism in solution is offered. © 2013 American Chemical Society.
• In-noi, O., Haller, K. J., Hall, G. B., Brezinski, W. P., Marx, J. M., Sakamoto, T., Evans, D. H., Glass, R. S., & Lichtenberger, D. L. (2014). Electrochemical, spectroscopic, and computational study of bis(μ-methylthiolato)diironhexacarbonyl: homoassociative stabilization of the dianion and a chemically reversible reduction/reoxidation cycle.. Organometallics, 33(18), 5009-5019.
• Lichtenberger, D. L. (2014). Organometallic Electrochemistry. Organometallics, 33(18).
• Lichtenberger, D. L. (2014). Spotlights on Recent JACS Publications. J. Am. Chem. Soc..
• Lichtenberger, D. L., & Gladysz, J. A. (2014). New Author Guidelines for 2014: A Format for Computational Structural Data That Can Be Opened with Freely Available Programs such as "Mercury".. Organometallics, 33(4), 835.
• Chiarella, G. M., Cotton, F. A., Durivage, J. C., Lichtenberger, D. L., & Murillo, C. A. (2013). Solubilizing the most easily ionized molecules and generating powerful reducing agents. Journal of the American Chemical Society, 135(47), 17889-17896.
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  Abstract: Two very soluble compounds having W2(bicyclic guanidinate) 4 paddlewheel structures show record low ionization energies (onsets at 3.4 to 3.5 eV) and very negative oxidation potentials in THF (-1.84 to -1.90 V vs Ag/AgCl). DFT computations show the correlation from the gas-phase ionization energies to the solution redox potentials and chemical behavior. These compounds are thermally stable and easy to synthesize in high yields and good purity. They are very reactive and potentially useful stoichiometric reducing agents in nonpolar, nonprotonated solvents. © 2013 American Chemical Society.
• Palmer, M. H., Hoffmann, S. V., Jones, N. C., Smith, E. R., & Lichtenberger, D. L. (2013). The electronic states of pyridine-N-oxide studied by VUV photoabsorption and ab initio configuration interaction computations. Journal of Chemical Physics, 138(21).
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  PMID: 23758381;Abstract: The first vacuum-ultraviolet absorption spectrum of pyridine-N-oxide has been obtained, and has led to the identification of nearly 30 Rydberg states. These states were identified by use of the vibrational envelope ("footprint") of the UV-photoelectron spectrum, and are based on the first to the third ionization energies (IE). The adiabatic IE order, central to the Rydberg state symmetry identification, is confirmed by multi-configuration SCF calculations as: 12B1 < 12B2 < 12A2 < 22B1. Several excited valence state equilibrium structures were determined by multi-configuration SCF and coupled cluster procedures. Multi-reference multi-root CI was used to calculate both Rydberg and valence state vertical excitation energies and oscillator strengths, which were correlated with the experimental measurements. © 2013 AIP Publishing LLC.
• Seidel, R. A., Hall, G. B., Swenson, M. T., Nichol, G. S., Lichtenberger, D. L., Evans, D. H., & Glass, R. S. (2013). Synthesis and characterization of [FeFe]-hydrogenase mimics appended with a 2-phenylazopyridine ligand. Journal of Sulfur Chemistry, 34(6), 566-579.
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  Abstract: Two new complexes in which 2-phenylazopyridine (pap) chelates iron in hydrogenase mimics, 1,2-(-benzenedithiolato)-2′- phenylazopyridinediirontetracarbonyl and 1,3-(-propanedithiolato)-2′- phenylazo- pyridinediirontetracarbonyl have been synthesized and fully characterized, including X-ray crystal structure determinations. The electronic structures of the two complexes are compared with the analogous 1,2-(-benzenedithiolato)diironhexacarbonyl and 1,3-(-propanedithiolato) diironhexacarbonyl complexes. Based on comparison of the crystal structures, the overall bonding in the 2Fe2S core of the molecules is little perturbed by replacing two carbonyl ligands with the pap ligand. Also, the coordinated pap ligand retains a similar structure and NN bond distance to that of the uncoordinated ligand. However, the charge asymmetry in the 2Fe2S core that results from chelating the pap ligand on one of the iron atoms induces substantial localization of the individual orbital characters in the 2Fe2S core. Most interesting, the pap-substituted complexes feature a novel strong long wavelength absorption in the visible region that imparts a deep blue color to the molecules. TDDFT calculations reveal the nature of this absorption as excitation to a low-lying empty orbital on the pap ligand mixed with filled primarily metal d orbitals of the 2Fe2S core. © 2013 Taylor & Francis.
• Gladysz, J. A., Bochmann, M., Gabbaï, F. P., Lichtenberger, D. L., Liebeskind, L. S., & Marks, T. J. (2012). The inaugural 2012 Organometallics symposium. Organometallics, 31(21), 7303-7305.
• Griffith, O. L., Anthony, J. E., Jones, A. G., Shu, Y., & Lichtenberger, D. L. (2012). Substituent effects on the electronic characteristics of pentacene derivatives for organic electronic devices: Dioxolane-substituted pentacene derivatives with triisopropylsilylethynyl functional groups. Journal of the American Chemical Society, 134(34), 14185-14194.
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  PMID: 22867002;Abstract: The intramolecular electronic structures and intermolecular electronic interactions of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m] -pentacene (TP-5 pentacene), and 2,2,10,10-tetraethyl-6,14-bis- (triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (EtTP-5 pentacene) have been investigated by the combination of gas-phase and solid-phase photoelectron spectroscopy measurements. Further insight has been provided by electrochemical measurements in solution, and the principles that emerge are supported by electronic structure calculations. The measurements show that the energies of electron transfer such as the reorganization energies, ionization energies, charge-injection barriers, polarization energies, and HOMO-LUMO energy gaps are strongly dependent on the particular functionalization of the pentacene core. The ionization energy trends as a function of the substitution observed for molecules in the gas phase are not reproduced in measurements of the molecules in the condensed phase due to polarization effects in the solid. The electronic behavior of these materials is impacted less by the direct substituent electronic effects on the individual molecules than by the indirect consequences of substituent effects on the intermolecular interactions. The ionization energies as a function of film thickness give information on the relative electrical conductivity of the films, and all three molecules show different material behavior. The stronger intermolecular interactions in TP-5 pentacene films lead to better charge transfer properties versus those in TIPS pentacene films, and EtTP-5 pentacene films have very weak intermolecular interactions and the poorest charge transfer properties of these molecules. © 2012 American Chemical Society.
• Head, A. R., Renshaw, S. K., Uplinger, A. B., Lomprey, J. R., Selegue, J. P., & Lichtenberger, D. L. (2015). Experimental measure of metal-alkynyl electronic structure interactions by photoelectron spectroscopy: (η5-C5H5)Ru(CO)2C≡CMe and [(η5-C5H5)Ru(CO)2]2(μ-C≡C).. Polyhedron, 86, 141-150.
• Meyer, G. J., Hall, G. B., Smith, E. R., Sakamoto, T., Lichtenberger, D. L., & Glass, R. S. (2015). Through space interaction between ferrocenes mediated by a thioether.. Polyhedron, 86, 125-132.
• Palmer, M. H., Camp, P. J., Hoffmann, S. V., Jones, N. C., Head, A. R., & Lichtenberger, D. L. (2012). The electronic states of 1,2,4-triazoles: A study of 1H- and 1-methyl-1,2,4-triazole by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy and a comparison with ab initio configuration interaction computations. Journal of Chemical Physics, 136(9).
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  PMID: 22401443;Abstract: The first vacuum ultraviolet absorption spectrum of a 1,2,4-triazole has been obtained and analyzed in detail, with assistance from both an enhanced UV photoelectron spectroscopic study and ab initio multi-reference multi-root configuration interaction procedures. For both 1H- and 1-methyl-1,2,4-triazoles, the first ionization energy bands show complex vibrational structure on the low-energy edges of otherwise unstructured bands. Detailed analysis of these bands confirms the presence of three ionized states. The 6-7 eV VUV spectral region shows an unusual absorption plateau, which is interpreted in terms of the near degeneracy of the first two ionization energies, leading to a pseudo Jahn-Teller effect. The fingerprint of the ionization spectrum yields band origins for several Rydberg states. The configuration interaction study shows that although the equilibrium structure for the first cation is effectively planar, the second cation shows significant twisting of the ring system. Some calculated singlet electronic states also show skeletal twisting in which the ring C-H is substantially out of plane. © 2012 American Institute of Physics.
• Van Dorn, L. O., Borowski, S. C., & Lichtenberger, D. L. (2015). From gas-phase ionization energies to solution oxidation potentials: Dimolybdenum tetraformamidinate paddlewheel complexes.. Inorganica Chimica Acta, 424, 316-321.
• Estes, D. P., Vannucci, A. K., Hall, A. R., Lichtenberger, D. L., & Norton, J. R. (2011). Thermodynamics of the metal-hydrogen bonds in (η⁵-C ₅H₅)M(CO)₂H (M = Fe, Ru, Os). Organometallics, 30(12), 3444-3447.
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  Abstract: Several different pKa values for the complex (η5-C5H5)Fe(CO)2H (FpH) in acetonitrile are present in the literature, ranging over 7 orders of magnitude. As a result, the energy of its Fe-H bond is also in dispute, making it difficult to predict the reactivity of this complex. The pKa, bond dissociation enthalpy, bond dissociation free energy, and hydricity of FpH have been measured. The pKa values of analogous group 8 hydrides have been determined, and the energies of the M-H bonds and the hydricity of FpH have been established. © 2011 American Chemical Society.
• Flores, J. A., Andino, J. G., Tsvetkov, N. P., Pink, M., Wolfe, R. J., Head, A. R., Lichtenberger, D. L., Massa, J., & Caulton, K. G. (2011). Assessment of the electronic structure of 2,2′-pyridylpyrrolides as ligands. Inorganic Chemistry, 50(17), 8121-8131.
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  PMID: 21776969;Abstract: The ligand class 2,2′-pyridylpyrrolide is surveyed, both for its structural features and its electronic structure, when attached to monovalent K, Cu, Ag, Au, and Rh. The influence of pyrrolide ring substituents is studied, as well as the question of push/pull interaction between the pyridyl and pyrrolide halves. The π donor ability of the pyrrolide is found to be less than that of an analogous phenyl. However, in contrast to the phenyl analog, the HOMO is pyrrolide π in character for pyridylpyrrolide complexes of copper and rhodium, while it is conventionally metal localized for planar, d 8 rhodium pyridylphenyl. Monovalent three-coordinate copper complexes show great deviations from Y-shaped toward T-shaped structures, including cases where the pyridyl ligand bonds only weakly. © 2011 American Chemical Society.
• Palmer, M. H., Hoffmann, S. V., Jones, N. C., Head, A. R., & Lichtenberger, D. L. (2011). The electronic states of 1,2,3-triazole studied by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy, and a comparison with ab initio configuration interaction methods. Journal of Chemical Physics, 134(8).
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  PMID: 21361541;Abstract: The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ 5s3p3d1f basis set and a set of Rydberg 4s3p3d3f functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (∼5.5-6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved resolution yielded adiabatic and vertical ionization energies and reorganization energies for several of the lowest cationic states. As well as excitations to the s, p, d-Rydberg states are the excitations consistent with an f-series. © 2011 American Institute of Physics.
• Wiebelhaus, N. J., Cranswick, M. A., Klein, E. L., Lockett, L. T., Lichtenberger, D. L., & Enemark, J. H. (2011). Metal-sulfur valence orbital interaction energies in metal-dithiolene complexes: Determination of charge and overlap interaction energies by comparison of core and valence ionization energy shifts. Inorganic Chemistry, 50(21), 11021-11031.
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  PMID: 21988484;PMCID: PMC3205315;Abstract: The electronic interactions between metals and dithiolenes are important in the biological processes of many metalloenzymes as well as in diverse chemical and material applications. Of special note is the ability of the dithiolene ligand to support metal centers in multiple coordination environments and oxidation states. To better understand the nature of metal-dithiolene electronic interactions, new capabilities in gas-phase core photoelectron spectroscopy for molecules with high sublimation temperatures have been developed and applied to a series of molecules of the type Cp 2M(bdt) (Cp = η 5-cyclopentadienyl, M = Ti, V, Mo, and bdt = benzenedithiolato). Comparison of the gas-phase core and valence ionization energy shifts provides a unique quantitative energy measure of valence orbital overlap interactions between the metal and the sulfur orbitals that is separated from the effects of charge redistribution. The results explain the large amount of sulfur character in the redox-active orbitals and the 'leveling' of oxidation state energies in metal-dithiolene systems. The experimentally determined orbital interaction energies reveal a previously unidentified overlap interaction of the predominantly sulfur HOMO of the bdt ligand with filled π orbitals of the Cp ligands, suggesting that direct dithiolene interactions with other ligands bound to the metal could be significant for other metal-dithiolene systems in chemistry and biology. © 2011 American Chemical Society.
• Bochmann, M., Brookhart, M., Gladysz, J. A., Lichtenberger, D. L., Liebeskind, L. S., Marks, T. J., Schrock, R. R., Sweigart, D. A., & Whitmire, K. H. (2010). Dedication to the special volume for Dietmar Seyferth. Organometallics, 29(21), 4647-.
• Cossairt, B. M., Cummins, C. C., Head, A. R., Lichtenberger, D. L., Berger, R. J., Hayes, S. A., Mitzel, N. W., & Gang, W. u. (2010). On the Molecular and Electronic Structures of AsP₃ and P ₄. Journal of the American Chemical Society, 132(24), 8459-8465.
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  PMID: 20515032;Abstract: The molecular and electronic structures of AsP3 and P 4 have been investigated. Gas-phase electron diffraction studies of AsP3 have provided rg bond lengths of 2.3041(12) and 2.1949(28) Å for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P 4 has been redetermined and provides an updated value of 2.1994(3) Å for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP 3 and P4 and shows that electronically AsP3 and P4 are quite similar. Solid-state 75As and 31P NMR spectroscopy demonstrate the plastic nature of AsP3 and P 4 as solids, and an extreme upfield 75As chemical shift has been confirmed for the As atom in AsP3. Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP 3 and P4 can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules. © 2010 American Chemical Society.
• Gladysz, J. A., Bochmann, M., Lichtenberger, D. L., Liebeskind, L. S., Marks, T. J., & Sweigart, D. A. (2010). Another great day for organometallic chemistry. Organometallics, 29(22), 5737-.
• Griffith, O. L., Anthony, J. E., Jones, A. G., & Lichtenberger, D. L. (2010). Electronic properties of pentacene versus triisopropylsilylethynyl- substituted pentacene: Environment-dependent effects of the silyl substituent. Journal of the American Chemical Society, 132(2), 580-586.
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  PMID: 20000766;Abstract: Energy measures of the intra- and intermolecular electronic effects of triisopropylsilylethynyl substitution on pentacene have been obtained from the combination of closely related gas phase and solid phase ultraviolet photoelectron spectroscopy (UPS) measurements along with solution electrochemical measurements. The results show that the shift to lower ionization energy that is expected with this substitution and observed in the gas phase measurements becomes negligible in solution and is even reversed in the solid phase. The principles that emerge from this analysis are supported by electronic structure calculations at the density functional theory level. The relation between the gas phase and solid phase UPS measurements illustrated here provides a general approach to investigating the electronic effects acting on molecules in the condensed phase, which in this case are greater than the direct substituent electronic effects within the molecule. Electronic properties such as lower ionization energies built into the single-molecule building blocks of materials and devices may be reversed in the solid state. © 2010 American Chemical Society.
• Griffith, O. L., Jones, A. G., Anthony, J. E., & Lichtenberger, D. L. (2010). Intermolecular effects on the hole states of triisopropylsilylethynyl- substituted oligoacenes. Journal of Physical Chemistry C, 114(32), 13838-13845.
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  Abstract: The effects of intermolecular interactions on the electronic properties of bis-triisopropylsilylethynyl-substituted (TIPS) anthracene, tetracene, and pentacene are obtained from comparison of the ionization energies measured by solid-phase ultraviolet photoelectron spectroscopy (UPS) with the ionization energies measured by gasphase UPS, and with the oxidation potentials measured electrochemically in solution. Additional insight is provided by electronic structure calculations at the density functional theory level. The results show that the solution-phase oxidation potentials correlate linearly with the gas-phase first ionization energies of TIPS oligoacenes, and both energies decrease with the increase in acene core size as expected for the increasing delocalization of the HOMO. However, the solid-phase ionization energies are independent of the acene core size, and thus do not follow the trend indicated by the molecular electronic structures and verified by the gas-phase and solution measurements. The solid-phase electronic properties such as charge injection barriers, ionization energies, and HOMO-LUMO energy gaps are greatly affected by the polarization effects of the surrounding molecules in the solid state, which dominate over the changes in molecular electronic properties caused by the change in acene core size. © 2010 American Chemical Society.
• Kuo, L. Y., Smith, C. P., Head, A. R., & Lichtenberger, D. L. (2010). Phosphonothioate hydrolysis through selective P-S bond scission by molybdenum metallocenes. Main Group Chemistry, 9(3-4), 283-295.
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  Abstract: We present an overview of the phosphonothioate hydrolytic chemistry promoted by molybdenum organometallics as well as new results on solvent effects. The metallocene bis(η 5-cyclopentadienyl) molybdenum(IV) dichloride (Cp2MoCl2 Cp=η5-C 5H5) and the methylated analog, (CH3Cp) 2MoCl2 hydrolyzes the compound O,S-diethyl phenylphosphonothioate (DEPP) whose core functional group mimics the neurotoxin VX. This is one of the few examples of phosphonothioate degradation yielding exclusively the desired P-S bond scission under mild aqueous conditions (pH 7, 25°C). The solvent composition affects the monomer-dimer equilibrium of aquated molybdocenes which in turn dictates the rate of DEPP hydrolysis. Kinetic and equilibrium results are presented to show that the monomer is the active species. Moreover, photoelectron spectroscopy data indicate that while the methylated (CH3Cp)2MoCl2 has a higher monomer concentration compared to Cp2MoCl2, the former has a diminished electrophilic molybdenum center that is key in phosphonothioate hydrolysis. © 2010 - IOS Press and the authors. All rights reserved.
• Lichtenberger, D., Griffith, O. L., Anthony, J. E., Jones, A. G., & Lichtenberger, D. L. (2010). Electronic properties of pentacene versus triisopropylsilylethynyl-substituted pentacene: environment-dependent effects of the silyl substituent. Journal of the American Chemical Society, 132(2).
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  Energy measures of the intra- and intermolecular electronic effects of triisopropylsilylethynyl substitution on pentacene have been obtained from the combination of closely related gas phase and solid phase ultraviolet photoelectron spectroscopy (UPS) measurements along with solution electrochemical measurements. The results show that the shift to lower ionization energy that is expected with this substitution and observed in the gas phase measurements becomes negligible in solution and is even reversed in the solid phase. The principles that emerge from this analysis are supported by electronic structure calculations at the density functional theory level. The relation between the gas phase and solid phase UPS measurements illustrated here provides a general approach to investigating the electronic effects acting on molecules in the condensed phase, which in this case are greater than the direct substituent electronic effects within the molecule. Electronic properties such as lower ionization energies built into the single-molecule building blocks of materials and devices may be reversed in the solid state.
• Chen, S., Chisholm, M. H., Davidson, E. R., English, J. B., & Lichtenberger, D. L. (2009). Theoretical and spectroscopic investigations of the bonding and reactivity of (RO)₃M≡N molecules, where M = Cr, Mo, and W. Inorganic Chemistry, 48(3), 828-837.
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  PMID: 19099428;Abstract: The electronic structures of the molecules (tBuO) 3M≡N (M = Cr, Mo, W) have been investigated with gas phase photoelectron spectroscopy and density functional calculations. It is found that the alkoxide orbitais mix strongly with the M≡N triple bond orbitais and contribute substantially to the valence electronic structure. The first ionization of (tBuO)3Cr≡N is from an orbital of a2(C3v) symmetry that is oxygen based and contains no metal or nitrogen character by symmetry. In contrast, the first ionizations of the molybdenum and tungsten analogues are from orbitais of a1 and e symmetry that derive from the highest occupied M≡N σ and π orbitais mixed with the appropriate symmetry combinations of the oxygen p orbitais. In this a1 orbital, the oxygen p orbitais mix with the highest occupied M≡N orbital of σ symmetry. This mixing reduces the metal character, consequently reducing the metal-nitrogen overlap interaction in this orbital. From computational modeling, the polarity of the M≡N bond increases down the group such that W≡N has the highest charge separation. In addition to investigation of the effects of the metals, the electronic influences of substitution at the alkoxide ligands have been examined for the molecules (RO)3Mo≡N (R = C(CH3)2H, C(CH3)3, and C(CH3)2CF3). The introduction of CF3 groups stabilizes the molecular orbital energies and increases the measured ionization energies, but does not alter the overall electronic structure. The bonding characteristics of the ( tBuO)3M≡N series are compared with those of organic nitriles. © 2009 American Chemical Society.
• Vannucci, A. K., Snyder, R. A., Gruhn, N. E., Lichtenberger, D. L., & Enemark, J. H. (2009). New insights into solvolysis and reorganization energy from gas-phase, electrochemical, and theoretical studies of Oxo-Tp Mo^v molecules. Inorganic Chemistry, 48(18), 8856-8862.
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  PMID: 19691275;PMCID: PMC2768231;Abstract: Molecules of the general form Tp*MoO(OR)2 [where Tp* = hydrotris(3,5-dimethyl-1 -pyrazolyl)borate and (OR)2 = (OMe)2, (OEt)2, and (O nPr)2 for alkoxide ligands and (OR)2 = O(CH2)3O, O(CH2)4O, and O[CH(CH 3)CH2CH(CH3)P for diolato ligands] were studied using gas-phase photoelectron spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations to examine the effect of increasing ligand size and structure on the oxomolybdenum core. Oxidation potentials and first ionization energies are shown to be sensitive to the character of the diolato and alkoxide ligands. A linear correlation between the solution-phase oxidation potentials and the gas-phase ionization energies resulted in an unexpected slope of greater than unity. DFT calculations indicated that this unique example of a system in which oxidation potentials are more sensitive to substitution than vertical ionization energies is due to the large differences in the cation reorganization energies, which range from 0.2 eV or less for the molecules with diolato ligands to around 0.5 eV for the molecules with alkoxide ligands. © 2009 American Chemical Society.
• Cozzolino, A. F., Gruhn, N. E., Lichtenberger, D. L., & Vargas-Baca, I. (2008). Valence electronic structure of benzo-2,1,3-chalcogenadiazoles studied by photoelectron spectroscopy and density functional theory. Inorganic Chemistry, 47(14), 6220-6226.
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  PMID: 18570410;Abstract: The He I photoelectron spectra of benzo-2,1,3-thia-, selena-, and telluradiazole were measured, and the observed ionization bands were assigned by comparison with the results of DFT calculations. Whereas the B3LYP exchange-correlation functional provided orbital energies that permitted a preliminary assignment by application of Koopman's theorem, a more-accurate interpretation was established by calculation of the vertical ionization energies with the PW91 functional and analysis of the correlation of energy levels along the homologous series. This strategy clarified earlier disagreements in the assignment of the spectrum of benzo-2,1,3-thiadiazole. © 2008 American Chemical Society.
• Cranswick, M. A., Gruhn, N. E., Enemark, J. H., & Lichtenberger, D. L. (2008). Electronic structure of the d¹ bent-metallocene Cp₂VCl₂: A photoelectron and density functional study. Journal of Organometallic Chemistry, 693(8-9), 1621-1627.
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  Abstract: The Cp2VCl2 molecule is a prototype for bent-metallocene complexes with a single electron in the metal d shell, but experimental measure of the binding energy of the d electron by photoelectron spectroscopy eluded early attempts due to apparent decomposition in the spectrometer to Cp2VCl. With improved instrumentation, the amount of decomposition is reduced and subtraction of ionization intensity due to Cp2VCl from the Cp2VCl2/Cp2VCl mixed spectrum yields the Cp2VCl2 spectrum exclusively. The measured ionization energies provide well-defined benchmarks for electronic structure calculations. Density functional calculations support the spectral interpretations and agree well with the ionization energy of the d1 electron and the energies of the higher positive ion states of Cp2VCl2. The calculations also account well for the trends to the other Group V bent-metallocene dichlorides Cp2NbCl2 and Cp2TaCl2. The first ionization energy of Cp2VCl2 is considerably greater than the first ionization energies of the second- and third-row transition metal analogues. © 2008 Elsevier B.V. All rights reserved.
• Cranswick, M. A., Gruhn, N. E., Oorhles-Steele, O., Ruddick, K. R., Burzlaff, N., Schenk, W. A., & Lichtenberger, D. L. (2008). Metal-sulfur dπ-pπ buffering of the oxidations of metal-thiolate complexes: Photoelectron spectroscopy of (η⁵-C₅H₅)Fe(CO)₂SR (SR = SCH₃, S^tBu) and (η⁵-C₅H₅)Re(NO)(PR₃)SCH₃ (PR₃ = PⁱPr₃, PPh₃). Inorganica Chimica Acta, 361(4), 1122-1133.
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  Abstract: The metal-sulfur bonding present in the transition metal-thiolate complexes CpFe(CO)2SCH3, CpFe(CO)2StBu, CpRe(NO)(PiPr3)SCH3, and CpRe(NO)(PPh3)SCH3 (Cp = η5-C5H5) is investigated via gas-phase valence photoelectron spectroscopy. For all four complexes a strong dπ-pπ interaction exists between a filled predominantly metal d orbital of the [CpML2]+ fragment and the purely sulfur 3pπ lone pair of the thiolate. This interaction results in the highest occupied molecular orbital having substantial M-S π* antibonding character. In the case of CpFe(CO)2SCH3, the first (lowest energy) ionization is from the Fe-S π* orbital, the next two ionizations are from predominantly metal d orbitals, and the fourth ionization is from the Fe-S π orbital. The pure sulfur pπ lone pair of the thiolate fragment is less stable than the filled metal d orbitals of the [CpFe(CO)2]+ fragment, resulting in a Fe-S π* combination that is higher in sulfur character than the Fe-S π combination. Interestingly, substitution of a tert-butyl group for the methyl group on the thiolate causes little shift in the first ionization, in contrast to the shift observed for related thiols. This is a consequence of the delocalization and electronic buffering provided by the Fe-S dπ-pπ interaction. For CpRe(NO)(PiPr3)SCH3 and CpRe(NO)(PPh3)SCH3, the strong acceptor ability of the nitrosyl ligand rotates the metal orbitals for optimum backbonding to the nitrosyl, and the thiolate rotates along with these orbitals to a different preferred orientation from that of the Fe complexes. The initial ionization is again the M-S π* combination with mostly sulfur character, but now has considerable mixing among several of the valence orbitals. Because of the high sulfur character in the HOMO, ligand substitution on the metal also has a small effect on the ionization energy in comparison to the shifts observed for similar substitutions in other molecules. These experiments show that, contrary to the traditional interpretation of oxidation of metal complexes, removal of an electron from these metal-thiolate complexes is not well represented by an increase in the formal oxidation state of the metal, nor by simple oxidation of the sulfur, but instead is a variable mix of metal and sulfur content in the highest occupied orbital. © 2007 Elsevier B.V. All rights reserved.
• Durivage, J. C., Gruhn, N. E., Bo, L. i., Dikarev, E. V., & Lichtenberger, D. L. (2008). The Electronic Structure and Bonding of the First p-Block Paddlewheel Complex, Bi₂(trifluoroacetate)₄, and Comparison to d-Block Transition Metal Paddlewheel Complexes: A Photoelectron and Density Functional Theory Study. Journal of Cluster Science, 1-20.
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  Abstract: The photoelectron spectrum and a density functional computational analysis of the first p-block paddlewheel complex, Bi2(tfa)4, where tfa = (O2CCF3)-, are reported. The photoelectron spectrum of Bi2(tfa)4 contains an ionization band between the region of metal-based ionizations and the region of overlapping ligand ionizations that is not seen in the photoelectron spectra of d-block paddlewheel complexes. This additional ionization arises from an a1g symmetry combination of the tfa ligand orbitals that is directed for σ bonding with the metals, and the unusual energy of this ionization follows from the different interaction of this orbital with the valence s and p orbitals of Bi compared to the valence d orbitals of transition metals. There is significant mixing between the Bi-Bi σ bond and this a1g M-L σ orbital. This observation led to a re-examination of the ionization differences between Mo2(tfa)4 and W2(tfa)4, where the metal-metal σ and π ionizations are overlapping for the Mo2 molecule but a separate and sharp σ ionization is observed for the W2 molecule. The coalescing of the σ and π bond ionizations of Mo2(tfa)4 is due to greater ligand orbital character in the Mo-Mo σ bond (∼7%) versus the W-W σ bond (∼1%). © 2008 Springer Science+Business Media, LLC.
• Griffith, O. L., Gruhn, N. E., Anthony, J. E., Purushothaman, B., & Lichtenberger, D. L. (2008). Electron transfer parameters of triisopropylsilylethynyl-substituted oligoacenes. Journal of Physical Chemistry C, 112(51), 20518-20524.
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  Abstract: Understanding the electronic properties and electron transfer characteristics of functionalized oligoacenes is of great importance for the fabrication of organic electronic devices. Charge transfer parameters of bis-triisopropylsilylethynyl-substituted anthracene, tetracene, and pentacene have been investigated based on the analysis of their ionization energies and radical cation reorganization energies. High-resolution gas-phase photoelectron spectroscopy measurements and first-principles quantum-mechanical calculations at the density functional theory level have been performed. The results indicate that the ionization energies in the gas phase and the inner-sphere reorganization energies are sensitive to the number of fused rings and the substitution by the triisopropylsilylethynyl (TIPS) group. This TIPS functional group shifts the first ionization band of these molecules to lower energy in the gas phase due to mixing between the frontier occupied orbitals of the TIPS group with the highest occupied acene orbital. This mixing adds geometry modifications of the TIPS substituents to those of the acene core that occur with ionization, resulting in a near doubling of the reorganization energies with electron transfer for these molecules. © 2008 American Chemical Society.
• Lichtenberger, D., Cranswick, M. A., Gruhn, N. E., Enemark, J. H., & Lichtenberger, D. L. (2008). Electronic Structure of the d Bent-metallocene Cp(2)VCl(2): A Photoelectron and Density Functional Study. Journal of organometallic chemistry, 693(8-9).
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  The Cp(2)VCl(2) molecule is a prototype for bent metallocene complexes with a single electron in the metal d shell, but experimental measure of the binding energy of the d electron by photoelectron spectroscopy eluded early attempts due to apparent decomposition in the spectrometer to Cp(2)VCl. With improved instrumentation, the amount of decomposition is reduced and subtraction of ionization intensity due to Cp(2)VCl from the Cp(2)VCl(2)/Cp(2)VCl mixed spectrum yields the Cp(2)VCl(2) spectrum exclusively. The measured ionization energies provide well-defined benchmarks for electronic structure calculations. Density functional calculations support the spectral interpretations and agree well with the ionization energy of the d(1) electron and the energies of the higher positive ion states of Cp(2)VCl(2). The calculations also account well for the trends to the other Group V bent metallocene dichlorides Cp(2)NbCl(2) and Cp(2)TaCl(2). The first ionization energy of Cp(2)VCl(2) is considerably greater than the first ionization energies of the second- and third-row transition metal analogues.
• Cranswick, M. A., Dawson, A., Jon, J., Gruhn, N. E., Lichtenberger, D. L., & Enemark, J. H. (2007). Photoelectron spectroscopy and electronic structure calculations of d ¹ vanadocene compounds with chelated dithiolate ligands: Implications for pyranopterin Mo/W enzymes. Inorganic Chemistry, 46(25), 10639-10646.
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  PMID: 18001112;PMCID: PMC2532989;Abstract: Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the electronic structures of open-shell bent vanadocene compounds with chelating dithiolate ligands, which are minimum molecular models of the active sites of pyranopterin Mo/W enzymes. The compounds Cp 2V(dithiolate) [where dithiolate is 1,2-ethenedithiolate (S 2C2H2) or 1,2-benzenedithiolate (bdt), and Cp is cyclopentadienyl] provide access to a 17-electron, d1 electron configuration at the metal center. Comparison with previously studied Cp 2M(dithiolate) complexes, where M is Ti and Mo (respectively d 0 and d2 electron configurations), allows evaluation of d0, d1, and d2 electronic configurations of the metal center that are analogues for the metal oxidation states present throughout the catalytic cycle of these enzymes. A "dithiolate-folding effect" that involves an interaction between the vanadium d orbitals and sulfur p orbitals is shown to stabilize the d1 metal center, allowing the d1 electron configuration and geometry to act as a low-energy electron pathway intermediate between the d0 and d2 electron configurations of the enzyme. © 2007 American Chemical Society.
• Lichtenberger, D., Felton, G. A., Vannucci, A. K., Chen, J., Lockett, L. T., Okumura, N., Petro, B. J., Zakai, U. I., Evans, D. H., Glass, R. S., & Lichtenberger, D. L. (2007). Hydrogen generation from weak acids: electrochemical and computational studies of a diiron hydrogenase mimic. Journal of the American Chemical Society, 129(41).
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  Extended investigation of electrocatalytic generation of dihydrogen using [(mu-1,2-benzenedithiolato)][Fe(CO)3]2 has revealed that weak acids, such as acetic acid, can be used. The catalytic reduction producing dihydrogen occurs at approximately -2 V for several carboxylic acids and phenols resulting in overpotentials of only -0.44 to -0.71 V depending on the weak acid used. This unusual catalytic reduction occurs at a potential at which the starting material, in the absence of a proton source, does not show a reduction peak. The mechanism for this process and structures for the intermediates have been discerned by electrochemical and computational analysis. These studies reveal that the catalyst is the monoanion of the starting material and an ECEC mechanism occurs.
• Rajapakshe, A., Basta, R., Arif, A. M., Ernst, R. D., & Lichtenberger, D. L. (2007). Silyl substitution effects on metal-pentadienyl bonding: Synthesis, structure, photoelectron spectroscopy, and electronic structure of a high-valent half-open zirconocene. Organometallics, 26(11), 2867-2871.
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  Abstract: The molecule Cp(3-Me3Si-6,6-dmch)ZrI2 (Cp = η5-cyclopentadienyl; 3-Me3Si-6,6-dmch = η5-3-trimethylsilyl-6,6-dimethylcyclohexadienyl) has been synthesized, and the molecular and electronic structures have been investigated. Photoelectron spectroscopy shows that substitution of a trimethylsilyl group in place of a hydrogen atom on the 6,6-dmch ligand destabilizes all of the ionizations of Cp(3-Me3Si-6,6-dmch)ZrI2 by ca. 0.1-0.2 eV compared to those of Cp(6,6-dmch)ZrI2. Density functional calculations accurately reproduce the experimental structure of the molecule and agree with the observed shift of all ionizations to lower energies. Interestingly, the ionizations are calculated to shift to lower energies without the expected increase in electron density at the metal center. This apparent contradiction is understood from analysis of one-center and two-center charge effects in the molecule. As a consequence of these charge effects, the silyl substitution makes the 3-Me3Si-6,6-dmch ligand a slightly better donor and, surprisingly, also a better acceptor ligand than the 6,6-dmch ligand. © 2007 American Chemical Society.
• Cai, S., Shokhireva, T. K., Lichtenberger, D. L., & Walker, F. A. (2006). NMR and EPR studies of chloroiron(iii) tetraphenyl-chlorin and its complexes with imidazoles and pyridines of widely differing basicities. Inorganic Chemistry, 45(9), 3519-3531.
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  PMID: 16634582;PMCID: PMC2504473;Abstract: The NMR and EPR spectra of two bisimidazole and three bispyridine complexes of tetraphenylchlorinatoiron(III), [(TPC)Fe(L)2]+ (L = Im-d4, 2-MeHIm, 4-Me2NPy, Py, and 4-CNPy), have been investigated. The full resonance assignments of the [(TPC)Fe(L) 2]+ complexes of this study have been made from correlation spectroscopy (COSY) and nuclear Overhauser enhancement spectroscopy (NOESY) experiments and Amsterdam density functional (ADF) calculations. Unlike the [(OEC)Fe(L)2]+ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44, 1890-1903), the NMR data for the [(TPC)Fe(L)2]+ complexes of this study indicate that the ground state is S = 1/2 for each bisligand complex, whereas a higher spin state was present at NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all [TPCFe(L)2]+ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons that are part of the π system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases with decreasing donor ability of the substituted pyridine ligands, with the nonhindered imidazole ligand having slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower (4-Me2NPyH+ pKa = 9.7, H2Im + pKa = 6.65 (adjusted for the statistical factor of 2 protons)). The temperature dependence of the chemical shifts of all but the 4-Me2NPy bisligand complexes studied over the temperature range of the NMR investigations shows that they have mixed (dxy) 2(dxz,dyz)3/(dxzd yz)4(dxy)1 electron configurations that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with an S = 3/2 excited state in each case that in most cases lies at more than kT at room temperature above the ground state. The observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicate that it has a pure (dxzdyz) 4(dxy)1 ground state and that it is ruffled, because ruffling mixes the a2u(π)-like orbital of the chlorin into the singly occupied molecular orbital (SOMO). This mixing accounts for the negative chemical shift of the pyrroline-H (-6.5 ppm at -40°C) and thus the negative spin density at the pyrroline-α-carbons, but the mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)2]+, whose pyrroline-H chemical shift is -36 ppm at 25°C (Simonneaux, G.; Kobeissi, M. J. Chem. Sac., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. © 2006 American Chemical Society.
• Cotton, F. A., Donahue, J. P., Gruhn, N. E., Lichtenberger, D. L., Murillo, C. A., Timmons, D. J., O., L., Villagrán, D., & Wang, X. (2006). Facilitating access to the most easily ionized molecule: An improved synthesis of the key intermediate, W₂(hpp)₄Cl₂, and related compounds. Inorganic Chemistry, 45(1), 201-213.
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  PMID: 16390057;Abstract: A far superior synthesis is reported for W2(hpp) 4Cl2, a key intermediate in the synthesis of the most easily ionized closed-shell molecule W2(hpp)4 (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2- a]pyrimidine). At 200°C, the one-pot reaction of the air-stable and commercially available compounds W(CO)6 and Hhpp in o-dichlorobenzene produces W2(hpp)4Cl2 in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)2(CO)4 or W 2(μ-CO)2(μ-hpp)2(η2-hpp) 2, which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W2(hpp)4Cl2 and W2(hpp) 4. Two additional derivatives, W2(hpp)4X 2 where X is PF6- or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W2(μ-CO)2(μ-hpp)2(η 2-hpp)2 and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W2(hpp)4(TFPB)2 in THF shows two reduction processes with an E1/2 of -0.97 V for the first and -1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W 2(hpp)42+ units in W2(hpp) 4X2 compounds show that the metal-metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W-W distances. The low-energy gas-phase ionizations of W 2(hpp)4 are also reported and discussed. © 2006 American Chemical Society.
• Cotton, F. A., Durivage, J. C., Gruhn, N. E., Lichtenberger, D. L., Murillo, C. A., O., L., & Wilkinson, C. C. (2006). Photoelectron spectroscopy and DFT calculations of easily ionized quadruply bonded Mo ₂⁴⁺ compounds and their bicyclic guanidinate precursors. Journal of Physical Chemistry B, 110(40), 19793-19798.
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  PMID: 17020363;Abstract: A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo 24+ units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo 2(TEhpp) 4 complex which has a broad first ionization band centered around 4.27 ± 0.03 eV and an ionization onset at the very low energy of 3.93 ± 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo-Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (η 5-C 9Me 7) 2Co and (η 5-C 5Me 5) 2Cr. © 2006 American Chemical Society.
• Janczyk, A., Lichtenberger, D. L., & Ziurys, L. M. (2006). Competition between metal-amido and metal-imido chemistries in the alkaline earth series: An experimental and theoretical study of BaNH. Journal of the American Chemical Society, 128(4), 1109-1118.
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  PMID: 16433526;Abstract: The pure rotational spectrum of BaNH in its X1∑+ ground electronic state has been recorded using millimeter/submillimeter direct absorption methods; data for the deuterium and barium 137 isotopomers have been measured as well. The molecules were produced by the reaction of ammonia or ND3 and barium vapor in the presence of a dc discharge. Transitions arising from the ground vibrational state and the excited vibrational bending (0110) and heavy atom stretching (100) modes were measured. The rotational spectrum indicates a linear structure, with B0(BaNH) = 7984.549 MHz and B0(BaND) = 7060.446 MHz. An rm(1) structure has been determined, yielding r(BaN) = 2.077 ± 0.002 Å and r(NH) = 1.0116 ± 0.0006 Å. Density functional calculations using an extensive Slater-type basis set with inclusion of scalar relativistic effects gives geometrical parameters and vibrational frequencies for BaNH in excellent agreement with those determined by experiment. The molecular orbital and natural bond order analyses of the BaNH wave function show Ba-N π bonds formed by electron donation from the formally filled N 2p orbitals of the imido group to the empty Ba 5d orbitals. The multiple bonding between Ba and N stabilizes the linear geometry and, along with the relative ease of oxidation of the Ba atom, favors formation of the metal-imido species over that of the metal-amido species that have been found from similar studies with Mg, Ca, and Sr atoms in this group. © 2006 American Chemical Society.
• Rajapakshe, A., Paz-Sandoval, M. A., Gutierrez, J. A., Navarro-Clemente, M. E., Saavedra, P. J., Gruhn, N. E., & Lichtenberger, D. L. (2006). Heteropentadienyl analogues of half-open ruthenocenes: Metal-ligand interactions and electronic structure perturbations. Organometallics, 25(8), 1914-1923.
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  Abstract: The electronic structures of molecules of the form Cp*Ru(η 5-Pdl) (where Cp* = η 5-pentamethylcyclopentadienyl and Pdl = 2,4-dimethylpentadienyl and various heteropentadienyl ligands, including the azapentadienyl ligand 1-tert-butyl-3,5-dimethyl-1-azapentadienyl and the oxopentadienyl ligands 2,4-dimethyl-1-oxopentadienyl and 2,4-di-tert-butyl-1-oxopentadienyl) have been investigated using photoelectron spectroscopy and computational methods. The photoelectron spectra of these half-open metallocenes allow direct insight into the bonding capabilities of pentadienyl ligands and the influences of heteroatom substitution on the electronic structure. The number of separate valence ionization bands that are observed corresponds directly to the number of occupied valence metal d orbitals and highest occupied π orbitals of the Cp* and Pdl ligands plus (in the case of the heteropentadienyl ligands) an ionization that derives from the heteroatom (oxygen or nitrogen) lone pair orbital that is in the plane of the ligand. The heteropentadienyl ligand substitution has strong effects on both the ligand- and the metal-based ionizations. Interestingly, the ease of oxidation of the molecules does not follow the expected periodic trend of increasing ionization energy with increasing electronegativity of heteroatom substitution. Density functional calculations give orbital energies and characters for the Cp*Ru(Pdl) molecules in good agreement with those determined by experiment and offer an explanation of the unusual trend in ionization energies with heteroatom substitution. The calculations also show enhanced Ru - cyclopentadienyl bonding accompanying a weakening of Ru - pentadienyl bonding as the pentadienyl ligand becomes more electronegative with heteroatom substitution, which is important for understanding the relative structures and chemical reactivities of heteropentadienyl - metal complexes. © 2006 American Chemical Society.
• Cai, S., Lichtenberger, D. L., & Walker, F. A. (2005). NMR and EPR studies of the bis(pyridine) and bis(tert-butyl isocyanide) complexes of iron(III) octaethylchlorin. Inorganic Chemistry, 44(6), 1890-1903.
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  PMID: 15762715;Abstract: The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L) 2]+ (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (dxy)2(dxzdyz)3 to (dxzdyz)4(dxy)1. The NMR data from 303 to 183 K show that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)2]+ complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large π spin density on the adjacent carbons which are part of the π system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Hückel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(II) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)2]+ looks somewhat like that of [(OEC)-Fe(4-Me2NPy)2]+, while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)-Fe(t-BuNC)2]+. The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)2]+ shows that it has a mixed (dxzdyz)4(dxy)1 and (dxy)2(dxz,dyz)3 electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the dyz orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the dxy orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm-1, explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the α-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)2]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both σ and π spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring. © 2005 American Chemical Society.
• Cotton, F. A., Donahue, J. P., Lichtenberger, D. L., Murillo, C. A., & Villagrán, D. (2005). Expeditious access to the most easily ionized closed-shell molecule, W ₂(hpp)₄. Journal of the American Chemical Society, 127(31), 10808-10809.
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  PMID: 16076168;Abstract: A new synthetic path, far superior to either of those previously available, to the W2(hpp)4 molecule (Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) is reported. The reaction of W(CO)6 with Hhpp in o-dichlorobenzene at 200 °C produces W2(hpp)4Cl2 in a one-pot reaction in over 90% yield. This compound is stable and easily stored for further use, and it can be efficiently reduced in a one-step reaction to the title compound W2(hpp)4. Copyright © 2005 American Chemical Society.
• Amashukeli, X., Gruhn, N. E., Lichtenberger, D. L., Winkler, J. R., & Gray, H. B. (2004). Inner-sphere electron-transfer reorganization energies of zinc porphyrins. Journal of the American Chemical Society, 126(47), 15566-15571.
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  PMID: 15563186;Abstract: Inner-sphere electron-transfer reorganization energies of Zn(protoporphyrin IX) and Zn(octaethylporphyrin) are determined from band-shape analyses of the first ionization obtained by gas-phase valence photoelectron spectroscopy. The experimentally determined total inner-sphere reorganization energies for self-exchange (120-140 meV) indicate that structural changes upon oxidation are largely confined to the porphyrin ring, and substituents on the ring or solvent and other environmental factors make smaller contributions. Computational estimates by different models vary over a wide range and are sensitive to numerical precision factors for these low reorganization energies. Of current computational models that are widely available and practical for molecules of this size, functionals that contain a mixture of Hartree-Fock exchange and DFT exchange-correlation appear to be the most applicable.
• Joshi, H. K., Arvin, M. E., Durivage, J. C., Gruhn, N. E., Carducci, M. D., Westcott, B. L., Lichtenberger, D. L., & Enemark, J. H. (2004). Photoelectron spectra of potassium salts of hydrotris(pyrazol-1-yl)borates: Electronic structure of the electron withdrawing scorpionates Tp ^(CF3)2, Tp*^Cl and comparison to Tp* and Tp. Polyhedron, 23(2-3), 429-438.
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  Abstract: The electronic structures of the potassium salts of the homoscorpionates hydrotris(3,5-dimethylpyrazol-1-yl)borate (Tp*, 1), hydrotris(4-chloro-3, 5-dimethylpyrazol-1-yl)borate (Tp*Cl, 2) and hydrotris(3,5-bis(trifluoromethyl)pyrazol-1-yl)borate (Tp(CF3)2, 3) are compared using gas-phase photoelectron spectroscopy and density functional theory (DFT). DFT calculations also are reported for the generic scorpionate potassium (hydrotris(pyrazol-1-yl)borate) (KTp). This is the first such experimental probe of the electronic structure of halogen containing scorpionate ligands and subtle differences in the ionizations from the frontier orbitals in the photoelectron spectra of 1 and 3 are observed that give insight into the influence of substituents upon metal-scorpionate bonding. Distinct assignments of the ionizations from the nitrogen σ-donor orbitals (σN) and σBH molecular orbitals are possible experimentally by the use of variable (He I and He II) excitation energies. The experimentally observed first ionization energy of 3 is stabilized by ∼2.0 eV relative to 1 due to the strong electron withdrawing effect of the trifluoromethyl groups. The photoelectron spectroscopic studies of NaTp (CF3)2 further confirm the assignments of ionizations from σN orbitals for 3 associated with the a and e sets in C 3 symmetry. The X-ray crystal structure of 2 as the (μ-aqua) 3(potassium hydrotris(4-chloro-3,5-dimethylpyrazol-1-yl)borate) 2 dimer is also reported. © 2003 Elsevier Ltd. All rights reserved.
• Rajapakshe, A., Gruhn, N. E., Lichtenberger, D. L., Basta, R., Arif, A. M., & Ernst, R. D. (2004). Pentadienyls vs cyclopentadienyls and reversal of metal-ligand bonding affinity with metal oxidation state: Synthesis, molecular structures, and electronic structures of high-valent zirconium pentadienyl complexes. Journal of the American Chemical Society, 126(43), 14105-14116.
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  PMID: 15506775;Abstract: Molecules of the form Cp(6,6-dmch)ZrX2 (Cp = η5-cyclopentadienyl, X = Cl, Br, I; 6,6-dmch = η5-6,6-dimethylcyclohexadienyl) have been synthesized, and the molecular and electronic structures have been investigated. These molecules allow direct comparison of the bonding and properties of pentadienyl and cyclopentadienyl ligands in the same high-oxidation-state metal complexes. Unlike the well-known Cp2ZrX2 analogues, these Cp(6,6-dmch)ZrX2 molecules are intensely colored, indicating significantly different relative energies of the frontier orbitals. Also unusual, the average Zr-C distances to the 6,6-dmch pentadienyl ligand are about 0.1 Å longer than the average Zr-C distances to the cyclopentadienyl ligand for these Zr(IV) complexes, opposite of what is observed for the Zr(II) complex Cp(2,6,6-tmch)Zr(PMe3)2 (tmch = η5-2,6,6-trimethylcyclohexadienyl), reflecting a dramatic reversal in the favorability of the bonding depending on the metal oxidation state. The experimental and computational results indicate that the color of the Cp(6,6-dmch)ZrX2 complexes is due to a 6,6-dmch ligand-to-metal charge-transfer band. Compared to the Cp2ZrX2 analogues, the Cp(6,6-dmch)ZrX2 molecules have a considerably less stable HOMO that is pentadienyl-based and an essentially unchanged metal-based LUMO. Also, the lowest unoccupied orbital of pentadienyl is stabilized relative to cyclopentadienyl and becomes a better potential delta electron acceptor, thus contributing to the differences in structure and reactivity of the low-valent and high-valent metal complexes.
• Joshi, H. K., Jon, J., Inscore, F. E., Gruhn, N. E., Lichtenberger, D. L., & Enemark, J. H. (2003). Investigation of metal-dithiolate fold angle effects: Implications for molybdenum and tungsten enzymes. Proceedings of the National Academy of Sciences of the United States of America, 100(7), 3719-3724.
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  PMID: 12655066;PMCID: PMC152988;Abstract: Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the interactions between the sulfur π-orbitals of arene dithiolates and high-valent transition metals as minimum molecular models of the active site features of pyranopterin Mo/W enzymes. The compounds (Tp*)MoO(bdt) (compound 1), Cp2Mo(bdt) (compound 2), and Cp2Ti(bdt) (compound 3) [where Tp* is hydrotris(3,5-dimethyl-1pyrazolyl)borate, bdt is 1,2-benzenedithiolate, and Cp is η5cyclopentadienyl] provide access to three different electronic configurations of the metal, formally d1, d2, and d0, respectively. The gas-phase photoelectron spectra show that ionizations from occupied metal and sulfur based valence orbitals are more clearly observed in compounds 2 and 3 than in compound 1. The observed ionization energies and characters compare very well with those calculated by density functional theory. A "dithiolate-folding-effect" involving an interaction of the metal in-plane and sulfur-π orbitals is proposed to be a factor in the electron transfer reactions that regenerate the active sites of molybdenum and tungsten enzymes.
• Lichtenberger, D. L. (2003). Electron distribution, bonding, and J(Si-H) NMR coupling constant in (η⁵-C₅H₅)(CO)₂ MnHSiCl₃: The molecular orbital view. Organometallics, 22(8), 1599-1602.
• Lichtenberger, D. L., Fan, H., & Gruhn, N. E. (2003). Ligand-mediated metal-metal interactions and localized versus delocalized mixed-valence cation states of biferrocene and bis(μ-fulvalenediyl)diiron characterized in the gas phase by valence photoelectron spectroscopy. Journal of Organometallic Chemistry, 666(1-2), 75-85.
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  Abstract: Gas-phase photoelectron spectroscopy is used to investigate metal-metal interactions and the mixed-valence positive ion states of biferrocene and bis(μ-fulvalenediyl)diiron. The spectra of phenylferrocene and 1,1′-diphenylferrocene a re used to show that, in comparison to ferrocene, the extension of the ligand π system and the reduced ligand symmetry do not have an appreciable effect on the band profile of the metal-based ionizations. In contrast, the initial ionization bands of both bimetallic molecules, which derive from the metal-based 2E2g ionizations of ferrocene, are spread over a wide energy range, indicating delocalization across the two metal halves of the molecule and formal oxidation states of +21/2 for each metal atom in these cation states. The broadening and splitting of this ionization band for bis(μ-fulvalenediyl)diiron is twice that observed for biferrocene, consistent with a through-bond ligand-mediated mechanism of interaction. Ionizations of the bimetallic molecules that derive from the metal-based2A1g ionizations of ferrocene occur in a single narrow band, indicating that both through-space and through-ligand interactions are not appreciable for the dz2-based orbitals. The difference between the metal-metal interactions in these positive ion states follows from the different overlap and energy match of the metal orbitals with fulvalendiyl orbitals of the appropriate symmetry. Most important to the metal-metal interaction in the ground ion state are empty fulvalendiyl orbitals with two nodes perpendicular to the C5 planes and gerade and ungerade symmetries with respect to the inversion centers of the molecules. In the gas phase, both species are found to be strongly interacting, delocalized mixed-valence compounds in their ground ion states. © 2002 Published by Elsevier Science B.V.
• Amashukeli, X., Winkler, J. R., Gray, H. B., Gruhn, N. E., & Lichtenberger, D. L. (2002). Electron-transfer reorganization energies of isolated organic molecules. Journal of Physical Chemistry A, 106(33), 7593-7598.
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  Abstract: He I photoelectron spectra of phenanthrene (1), 1,10-phenanthroline (2), phenazine (3), dibenzo[a,c]anthracene (4), dibenzo[a,c]phenazine (5), and dipyrido[3,2-a;2′3′-c]phenazine (6) have been obtained. Assignment of the π ionization states was aided by electronic structure calculations: the first ionization state of 1, 2B1(π1), is observed at 7.888 ± 0.002 eV, 2B2(π1) of 2 is at 8.342 ± 0.002 eV, and 2B1g(π1) of 3 is at 8.314 ± 0.002 eV. Spectra of 4-6 are reported for the first time: 2A2(π1) of 4 is at 7.376 ± 0.002 eV, and both 5 (7.983 ± 0.002 eV) and 6 (8.289 ± 0.002 eV) exhibit quasi-degenerate first and second ionization states. Quantum-mechanical reorganization energies, λQM, were extracted from analyses of vibrational structure: values are 149 ± 5 (1), 167 ± 5 (2), 68 ± 2 (3), and 92 ± 4 (4) meV. Low-frequency modes were treated semiclassically: values of λSC are estimated to be 21 ± 1 (1), 13 ± 1 (2), 22 ± 1 (3), 66 ± 1 (4), 27 ± 9 (5), and 16 ± 1 (6) meV. Reorganization energies (λ = λQM + λSC) of isolated molecules are 170 ± 5 (1), 180 ± 5 (2), 90 ± 2 (3), and 158 ± 4 (4) meV. Density functional calculations (B3LYP/6-311G++(d,p)) give λ values that are on average 63 meV lower than experimentally derived energies.
• Cotton, F. A., Gruhn, N. E., Jiande, G. u., Huang, P., Lichtenberger, D. L., Murillo, C. A., O., L., & Wilkinson, C. C. (2002). Closed-shell molecules that ionize more readily than cesium. Science, 298(5600), 1971-1974.
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  PMID: 12471252;Abstract: We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies.
• K., J., Gruhn, N. E., & Lichtenberger, D. L. (2002). Transferability of the valence ionization intensities of chemical functional groups between molecules. Journal of Physical Chemistry A, 106(42), 9999-10009.
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  Abstract: The transferability of the valence ionization intensities of chemical functional groups is investigated with three common laboratory ionization sources (Ne Iα, 16.7 eV; He Iα, 21.2 eV; and He IIα, 40.8 eV) for a series of molecules. Each molecule contains two different functional groups that are well separated electronically and spatially by an alkane chain linker. The optimum length of the alkane chain between the functional groups is investigated. In this initial study, the functional groups are RCl, RBr, RSH, and RFc, where R is an alkane chain and Fc is ferrocene with an alkane chain bonded to one cyclopentadienyl ring. An additional feature of this study is that each functional group contains a molecular orbital with nearly pure atomic character, allowing comparison to theoretical atomic ionization cross-section ratios. In general, the observed photoelectron intensity changes are more gradual than those predicted by either theoretical atomic photoionization cross sections or by a complete Gelius analysis based on molecular orbital compositions obtained from electronic structure calculations. Experimentally, there is a high degree of transferability of the relative ionization intensities of the chemical functional groups between the molecules of this set. These results indicate that an empirical library of the relative ionization cross sections of chemical functional groups as a function of photon energy will be a useful aid to the study of more complex molecules and will provide an experimental foundation for further theoretical studies of molecular photoelectron cross sections.
• Lichtenberger, D. L., Gruhn, N. E., Rai-Chaudhuri, A., Renshaw, S. K., Gladysz, J. A., Jiao, H., Seyler, J., & Igau, A. (2002). How do the electronic structures of low-symmetry metal-hydride and -alkyl complexes compare? Photoelectron spectroscopy and computational studies of (η⁵-C₅R₅)Re(NO)(L)R′(L = CO, P(C₆H₅)₃; R, R′ = H, CH₃). Organometallics, 21(25), 5494-5504.
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  Abstract: The electronic structures of CpRe(NO)(L)R and Cp*Re(NO)(L)R (Cp = *5-C5H5, Cp* = η5-C5(CH3)5; L = CO, P(C6H5)3; R = H, CH3) are studied using gas-phase photoelectron spectroscopy and density functional theory. Separate valence ionizations from the three occupied metal-based orbitals of the d6 Re center, the Re-R σ bond orbitals, and the predominantly Cp e1″ pπ orbitals are clearly observed. Comparison of the shapes and energies of the Cp and σ(Re-R) ionizations indicates an additional direct interaction between these orbitals that is sensitive to energy matching. This interaction results in a more delocalized σ-bonding framework for the methyl complexes than for the analogous hydrides and halides. The energy shifts and cross-sections of the metal-based ionizations provide quantitative measures of the different abilities of the nitrosyl, carbonyl, and phosphine ligands to delocalize and stabilize the metal electron density through π back-bonding. In these molecules the stabilization of a metal-based ionization by an NO ligand (∼1.4 eV) is about twice that by a CO ligand (∼0.7 eV), which is in turn about twice that by a P(C6H5)3 ligand (∼0.4 eV). The shifts of the metal-based ionization energies when the hydride ligand is replaced by methyl show that the methyl ligand is acting as a weak π donor. The first metal-based ionization shifts more than the second upon substitution of methyl for hydride, because it is less delocalized and consequently has more metal character for π interaction with the R ligand. This difference in the two metal π orbital distributions, along with the differences in energy, influences the rotational orientation of ligands at this site. The extent of this π interaction is sensitive to the electron richness at the metal center.
• Lichtenberger, D. L., Gruhn, N. E., Rai-Chaudhuri, A., Renshaw, S. K., Gladysz, J. A., Jiao, H., Seyler, J., & Igau, A. (2002). Vibrational progressions in the valence ionizations of transition metal hydrides: Evaluation of metal-hydride bonding and vibrations in (η⁵-C₅R₅)Re(NO)(CO)H [R = H, CH₃]. Journal of the American Chemical Society, 124(7), 1417-1423.
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  PMID: 11841310;Abstract: The first examples of vibrational structure in metal-ligand σ-bond ionizations are observed in the gas-phase photoelectron spectra of CpRe(NO)(CO)H and Cp*Re(NO)(CO)H [Cp = η5-C5H5, Cp* = η5-C5(CH3)5]. The vibrational progressions are due to the Re-H stretch in the ion states formed by removal of an electron from the predominantly Re-H σ-bonding orbitals. A vibrational progression is also observed in the corresponding ionization of the deuterium analogue, Cp*Re(NO)(CO)D, but with lower vibrational energy spacing as expected from the reduced mass effect. The vibrational progressions in these valence ionizations are directly informative about the nature of the metal-hydride bonding and electronic structure in these molecules. Franck-Condon analysis shows that for these molecules the Re-H or Re-D bond lengthens by 0.25(1) Å when an electron is removed from the Re-H or Re-D σ-bond orbital. This bond lengthening is comparable to that of H2 upon ionization. Removal of an electron from the Re-H or Re-D bonds leads to a quantum-mechanical inner sphere reorganization energy (λQM) of 0.34(1) eV. These observations suggest that even in these low symmetry molecules the orbital corresponding to the Re-H σ bond and the Re-H vibrational mode is very localized. Theoretical calculations of the electronic structure and normal vibrational modes of CpRe(NO)(CO)H support a localized two-electron valence bond description of the Re-H interaction.
• Lichtenberger, D. L., Fan, H., Gruhn, N. E., Bitterwolf, T. E., & Gallagher, S. (2000). Electronic structure of early transition-metal carbonyls: Gas-phase photoelectron spectroscopy of (η⁵-C₅H₅)M(CO)₄ (M = V, Nb, Ta). Organometallics, 19(10), 2012-2021.
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  Abstract: Gas-phase photoelectron spectroscopy is used to investigate the bonding between early transition metals and carbonyl and cyclopentadienyl ligands for the molecules (η5-C5H4R)M-(CO)4 (R = H, M = V, Nb, Ta; R = SiMe3, M = Nb, Ta; R = COCH3, M = Nb). The lowest ionization energy region contains two overlapping ionizations that arise from the two orbitals that are occupied according to the formal d4 metal configuration. However, the character of these ionizations is dominated by the carbonyls rather than by the metals, as evidenced by the extensive C-O stretching vibrational progressions observed with these ionizations, by the trends in the ionization cross sections between the molecules and with different ionization sources, and by the relative lack of shifts of these ionizations with metal substitution from vanadium to niobium to tantalum or with trimethylsilyl and acetyl substitutions on the cyclopentadienyl. The second group of ionizations for these molecules corresponds to orbitals with predominantly cyclopentadienyl π character that donate to empty metal d orbitals. A much larger shift of these ionizations is observed upon cyclopentadienyl substitution. The molecular structures are sensitive to the electron configurations. Both density functional theory and ab initio calculations reproduce well the geometry of the neutral molecules and also predict the geometry changes upon ionization. The first ionization, which relates to an orbital with the a1 symmetry of the metal dz2 orbital, is broad due to a substantial geometry change upon removal of an electron from this orbital. The shoulder on the cyclopentadienyl-based ionizations relates to a dynamic Jahn-Teller geometrical distortion. The unusually large metal-to-carbonyl back-bonding observed in these molecules is facilitated by the interligand overlap between the four carbonyls, which substantially stabilizes the appropriate symmetry-adapted carbonyl π* acceptor orbitals. The extensive carbonyl character in the valence electronic structure diminishes any trends in properties with substitutions of the metals down the group or with substitutions on the cyclopentadienyl ring.
• Lichtenberger, D. L., Pollard, J. R., Lynn, M. A., Cotton, F. A., & Feng, X. (2000). Metal-metal bonding in Rh₂(O₂CCF₃)₄: Extensive metal-ligand orbital mixing promoted by filled fluorine orbitals. Journal of the American Chemical Society, 122(13), 3182-3190.
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  Abstract: He I and He II gas-phase photoelectron spectra of Rh2(O2CCF3)4 are reported. The electron configuration of the metal-metal bond of Rh2(O2CCF3)4 is determined to be σ2 π4 δ2 δ*2 π*4 with an ionization energy order of σ ≃ π > δ > δ* ≃ π*. The δ* and π* ionization energies are similar within the range of vibrational energy separations. Assignment of the Rh-Rh δ ionization is assisted by previous observations that ionizations from δ orbitals in M2(O2CCF3)4 (M = Mo, W) and Mo2(O2CH)4 show enhanced intensity over ionizations from the σ and π orbitals with He II excitation. Changes in ionization energies from the dimolybdenum molecule to the dirhodium molecule and changes in ionization intensities from He I to He II excitation indicate greater metal-ligand mixing in these molecules than observed in other dimetal tetracarboxylates. Amsterdam density functional calculations agree with the observation that the Rh-Rh δ* and π* energies are similar. The calculations also indicate substantial ligand mixing into the metal-metal σ and π orbitals, which is enhanced by the trifluoroacetate ligand despite the inductive withdrawal of electron density by the electronegative fluorine atoms. It is found that a specific set of ligand orbitals that possess the same symmetries as the Rh-Rh σ and π orbitals are destabilized by overlap interactions with the filled fluorine p orbitals, resulting in greater metal - ligand mixing among these orbitals. The increased mixing explains the long-observed enhancement of the δ and δ* ionizations from He I to He II excitation for a number of M2(O2CCF3)4 (M = Mo, W, Ru) systems.
• Lynn, M. A., & Lichtenberger, D. L. (2000). Comparison of the Bonding of Benzene and C₆₀ to a Metal Cluster: Ru₃(CO)₉(μ³-η²,η ²,η²-C₆H₆) and Ru₃(CO)₉(μ³-η²,η ²,η²-C₆₀). Journal of Cluster Science, 11(1), 169-188.
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  Abstract: The electron distributions and bonding in Ru3(CO)9(μ3-η2,η 2,η2-C6H6) and Ru3(CO)9(μ3-η2,η 2,η2-C60) are examined via electronic structure calculations in order to compare the nature of ligation of benzene and buckminsterfullerene to the common Ru3(CO)9 inorganic cluster. A fragment orbital approach, which is aided by the relatively high symmetry that these molecules possess, reveals important features of the electronic structures of these two systems. Reported crystal structures show that both benzene and C60 are geometrically distorted when bound to the metal cluster fragment, and our ab initio calculations indicate that the energies of these distortions are similar. The experimental Ru-Cfullerene bond lengths are shorter than the corresponding Ru-Cbenzene distances and the Ru-Ru bond lengths are longer in the fullerene-bound cluster than for the benzene-ligated cluster. Also, the carbonyl stretching frequencies are slightly higher for Ru3(CO)9(μ3-η2,η 2,η2-C60) than for Ru3(CO)9(μ3-η2,η 2,η2-C6H6). As a whole, these observations suggest that electron density is being pulled away from the metal centers and CO ligands to form stronger Ru-Cfullerene than Ru-Cbenzene bonds. Fenske-Hall molecular orbital calculations show that an important interaction is donation of electron density in the metal-metal bonds to empty orbitals of C60 and C6H6. Bonds to the metal cluster that result from this interaction are the second highest occupied orbitals of both systems. A larger amount of density is donated to C60 than to C6H6, thus accounting for the longer metal-metal bonds in the fullerene-bound cluster. The principal metal-arene bonding modes are the same in both systems, but the more band-like electronic structure of the fullerene (i.e., the greater number density of donor and acceptor orbitals in a given energy region) as compared to C6H6 permits a greater degree of electron flow and stronger bonding between the Ru3(CO)9 and C60 fragments. Of significance to the reduction chemistry of M3(CO)9(μ3-η2,η 2,η2-C60) molecules, the HOMO is largely localized on the metal-carbonyl fragment and the LUMO is largely localized on the C60 portion of the molecule. The localized C60 character of the LUMO is consistent with the similarity of the first two reductions of this class of molecules to the first two reductions of free C60. The set of orbitals above the LUMO shows partial delocalization (in an antibonding sense) to the metal fragment, thus accounting for the relative ease of the third reduction of this class of molecules compared to the third reduction of free C60.
• Smith, K. J., Ondracek, A. L., Gruhn, N. E., Lichtenberger, D. L., Fanwick, P. E., & Walton, R. A. (2000). A comparative study of the isomers of ReOCl₃(PMe₃)₂ and ReOCl₃(PEt₃)₂. The isolation and characterization of ReH₇(PR₃)₂ and ReO(OEt)Cl₂(PR₃)₂ (R = Me or Et) and the photoelectron spectrum of ReH₇(PMe₃)₂. Inorganica Chimica Acta, 300-302, 23-31.
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  Abstract: The reactions of mer-trans-ReOCl3(PPh3)2 with PMe3 and PEt3 provide a route to the isomers fac-cis-ReOCl3(PR3)2 and mer-trans-ReOCl3(PR3)2 (R = Me or Et) which can be converted to ReH7(PR3)2 (R = Me or Et) in ca. 60% yield by reaction with LiAlH4 in THF and subsequent hydrolysis of the reaction mixtures. The properties of these heptahydrides have been examined, including studies of the remarkable stability of aqueous solutions of ReH7(PMe3)2 and the measurement of the gas-phase He(I) photoelectron spectrum of this complex. While the fac-cis and mer-trans isomers of ReOCl3(PR3)2 (R = Me or Et) do not convert to ReO(OEt)Cl2(PR3)2 upon reaction with ethanol, these ethoxides can be prepared by the reaction of mer-trans-ReO(OEt)Cl2(PPh3)2 with PMe3 and PEt3. The reactions of the isomers of ReOCl3(PMe3)2 with ethylamine, followed by the treatment of the reaction products with acetone, afford the salt [Me3PCMe2CH2C(O)CH3]ReO4. The compounds fac-cis-ReOCl3(PEt3)2, trans-ReO(OEt)Cl2(PMe3)2 and [Me3PCMe2CH2C(O)CH3]ReO4 have been structurally characterized by X-ray crystallography. (C) 2000 Elsevier Science S.A.
• Westcott, B. L., Gruhn, N. E., Michelsen, L. J., & Lichtenberger, D. L. (2000). Experimental observation of non-aufbau behavior: Photoelectron spectra of vanadyloctaethylporphyrinate and vanadylphthalocyanine [7]. Journal of the American Chemical Society, 122(33), 8083-8084.
• Gruhn, N. E., Lichtenberger, D. L., Ogura, H., & Walker, F. A. (1999). Reevaluation of the gas-phase valence photoelectron spectra of octaethylporphyrin and tetraphenylporphyrin. Inorganic Chemistry, 38(18), 4023-4027.
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  Abstract: Gas-phase valence photoelectron spectra are reported for 2,3,7,8,12,13,17,18-octaethylporphyrin (OEPH2) and 5,10,15,20-tetraphenylporphyrin (TPPH2). Comparison of spectra collected with He I and He II sources gives an empirical assignment of the relative order of the low-energy 2Au and 2B1u ion states in the gas phase. For OEPH2, the first ionization at 6.24 eV is assigned to the 2Au ion state, with the 2B1u ionization located 0.25 eV to higher energy. The photoelectron spectrum reported here is quite different from the previously reported spectrum of OEPH2. The first ionization of TPPH2 is assigned to the 2B1u ion state, with the 2Au ionization 0.27 eV to higher energy. The reversal of the ordering of the first two ionizations of OEPH2 and TPPH2 is due to the different site of the substituents and the different nodal properties of the porphyrin Orbitals. © 1999 American Chemical Society.
• Lichtenberger, D. L., Lynn, M. A., & Chisholm, M. H. (1999). Quadruple metal-metal bonds with strong donor ligands. Ultraviolet photoelectron spectroscopy of M₂(form)₄ (M = Cr, Mo, W; form = N,N'- diphenylformamidinate). Journal of the American Chemical Society, 121(51), 12167-12176.
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  Abstract: The He I photoelectron spectra of M2(form)4 (M = Cr, Mo, W; form = N,N'-diphenylformamidinate) and Mo2(cyform)4 (cyform = N,N'- dicyclohexylformamidinate) are presented. For comparison, the Ne I, He I, and He II photoelectron spectra of Mo2(p-CH3-form)4 have also been obtained. The valence ionization features of these molecules are interpreted based on (1) the changes that occur with the metal and ligand substitutions, (2) the changes in photoelectron cross sections with excitation source, and (3) the changes from previously studied dimetal complexes. These photoelectron spectra are useful for revealing the effects that better electron donor ligands have on the valence electronic structure of M2(L-L)4 systems. Comparison with the He I spectra of the isoelectronic M2(O2CCH3)4 compounds is particularly revealing. Unlike with the more electron- withdrawing acetate ligand, several formamidinate-based ionizations derived from the nitrogen pπ orbitals occur among the metal-metal σ, π, and δ ionization bands. Although these formamidinate-based levels are close in energy to the occupied metal-metal bonds, they have little direct mixing interaction with them. The shift of the metal-metal bond ionizations to lower ionization energies for the formamidinate systems is primarily a consequence of the lower electonegativity of the ligand and the better π donation into empty metal levels. The metal-metal δ orbital experiences some additional net bonding interaction with ligand orbitals of the same symmetry. Also, an additional bonding interaction from ligand-to-metal electron donation to the σd* orbital is identified. These spectra suggest a greater degree of metal- ligand covalency than in the related M2(O2CCH3)4 systems. Fenske-Hall molecular orbital and density functional (ADF) calculations agree with the assignment and interpretation of these spectra. Calculated ionization energies are reported for M2(form)4 based on several different density functionals and with different orientations and substitutions for the phenyl rings. It is found that good estimates of the ionization energies are obtained when the truncated system M2(HN(CH)NH)4, in which the phenyl groups are replaced by hydrogen atoms, is employed.
• Budzichowski, T. A., Chisholm, M. H., Tiedtke, D. B., Gruhn, N. E., & Lichtenberger, D. L. (1998). Preparation, characterization and electronic structure of W₂(NMe₂)₂(OR_f)₄, where R_f = CMe₂CF₃, CMe(CF₃)₂ and C(CF₃)₃, as deduced by photoelectron spectroscopic studies and the single crystal X-ray structure for Rf = OCMe(CF₃)₂. Polyhedron, 17(5-6), 705-711.
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  Abstract: The compounds W2(NMe2)2(OCMe2CF3) 4, 1, W2(NMe2)2(OCMe(CF3) 2)4, 2, and W2(NMe2)2 (OC(CF3)3)4, 3, have been obtained from reactions between W2(NMe2)6 and the respective fluoroalcohol in hydrocarbon solvents. The compounds 2 and 3 appear inert to further reaction with their fluoroalcohol at room temperature while 1 reacts further to give W2(OCMe2CF3)6. In solution NMR data indicate that all three compounds exist in a mixture of anti-and gauche-rotamers with restricted rotations about both W - N and W≡W bonds on the NMR time-scale. The solid-state molecular structure of 2 reveals a centrosymmetric NO2W≡WO2N core with W - W = 2.3107(6) Å, W - N = 1.914(4) Å, W - O = 1.959(3) and 1.907(3) Å and angles W - W - O/N within the range 100 to 110°. The gas phase photoelectron spectra for 1, 2 and 3 were obtained and are compared with that for W2(NMe2)6. The introduction of the fluoroalkoxide ligands has a pronounced effect in stabilizing the metal and nitrogen lone-pair based ionizations, and this effect is amplified by the successive replacement of each methyl group by a trifluoromethyl group. Also for the compounds 1, 2, and 3 the lowest ionization bands clearly reveal the removal of the degeneracy of the W - W π MOs as expected for anti and gauche complexes. © 1998 Elsevier Science Ltd. All rights reserved.
• Lichtenberger, D. L., Elkadi, Y., Gruhn, N. E., Hughes, R. P., Curnow, O. J., & Zheng, X. (1997). Electronic structure perturbations of substituted ruthenocenes: The first photoelectron spectra of perchloro- and perfluorocyclopentadienyl complexes. Organometallics, 16(24), 5209-5217.
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  Abstract: The electronic structure perturbations caused by cyclopentadienyl substitutions in the series of complexes (η5-C5H5)2Ru, (η5-C5Me5)(η5-C 5H5)Ru, (η5-C5Me5)2Ru, (η5-C5Me5)(η5-C 5-Cl5)Ru, and (η5-C5Me5)(η5-C 5F5)Ru are measured by gas-phase photoelectron spectroscopy. The shifts of the valence metal- and cyclopentadienyl-based ionizations give an indication of the overall electronic effects of methyl and halogen substitutions on the cyclopentadienyl rings. The halogen substituent interaction is an admixture of inductive σ-electron-withdrawing and filled-filled π-electron-overlap effects, which act in opposite directions. The π-overlap interaction is relatively weak in the case of chlorine substitution for hydrogen, and the combined σand π interactions give rise to an overall withdrawal of electron density from the metal center and increase in the metal d-based ionization energies. Fluorine substituents on cyclopentadienyl make the ring only slightly more electron withdrawing than η5-C5Cl5, despite the much greater electronegativity of fluorine compared to chlorine. The electron withdrawing ability of η5-C5F5 is tempered by the greater filled-filled interaction of the fluorine pπ orbitale with the cyclopentadienyl pπ orbitals, which lessens the stabilization of these orbitals and the withdrawal of electron density from the metal. It is interesting that in each case the metal d-based ionizations are stabilized more than the cyclopentadienyl π-based ionizations with halogen substitution for hydrogen, such that these ionizations begin to merge in the lowest ionization energy band.
• Lichtenberger, D. L., Gruhn, N. E., & Renshaw, S. K. (1997). Relative bonding capabilities of molecules to metals as measured by gas-phase photoelectron spectroscopy. Journal of Molecular Structure, 405(1), 79-86.
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  Abstract: The gas-phase photoelectron spectroscopy of compounds of the general form CpM(CO)2L (Cp = η5-C5H5; M = Cr, Mn, or Fe; L = ligand of interest) provides a direct experimental measure of the relative bonding capabilities of ligands to metals. The metal-based ionization energies of the formal d6 metal centers of these compounds are sensitive to the electron donation/acceptance of the ligands, and the splitting patterns of these ionizations are sensitive to the relative π-donation/acceptance properties of the ligands. This is the only experimental technique that can separate the ligand π-bonding effects from the σ-bonding and charge potential effects. A general formula is described by which the π-bonding capabilities of ligands can be compared.
• Lichtenberger, D. L., Gruhn, N. E., Rempe, M. E., Geiger, W. E., & Chin, T. T. (1995). Ligand-mediated metal-metal interactions in (ν⁵:ν⁵-fulvalene) bis (dicarbonylcobalt) and rhodium complexes. Inorganica Chimica Acta, 240(1-2), 623-629.
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  Abstract: Gas phase photoelectron spectroscopy (PES) is used to investigate the bonding and electronic structure in (fv) [M(CO)2]2 (fv = fulvalene, η5:η5-C10H82-; M = Co, Rh). The results for these bimetallic complexes are also compared to those for the analogous monometallic complexes CpM(CO)2 (Cp = η5-C5H5-; M = Co, Rh) which have been reported previously. The low valence ionization patterns observed for CpCo(CO)2 and (fv)[Co(CO)2]2 are very similar, indicating that there is little electronic interaction between the two metals of the dicobalt complex. The spectrum of (fv)[Rh(CO)2]2 also is very similar to the spectrum of CpRh(CO)2, except that the first metal ionizations in the bimetallic rhodium compound show a significant splitting (0.45 eV). This splitting is due to electronic interaction between the two metal centers which occurs via communication through the fulvalene π system. The differences in electronic structure are compared to the differences in electrochemical behavior of the Co and Rh fulvalene complexes. © 1995.
• Campbell, S., Marzluff, E. M., Rodgers, M. T., Beauchamp, J. L., Rempe, M. E., Schwinck, K. F., & Lichtenberger, D. L. (1994). Proton affinities and photoelectron spectra of phenylalanine and N-methyl- and N,N-dimethylphenylalanine. Correlation of lone pair ionization energies with proton affinities and implications for N-methylation as a method to effect site specific protonation of peptides. Journal of the American Chemical Society, 116(12), 5257-5264.
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  Abstract: A Fourier transform ion cyclotron resonance (FT-ICR) technique for measuring gas-phase proton affinities is presented which utilizes collisional dissociation of proton-bound clusters by off-resonance translational excitation. A simplified RRKM analysis relates unimolecular dissociation rates to proton affinities. This technique is used to measure values for the proton affinities of phenylalanine and N-methyl- and N,N-dimethylphenylalanine of 220.3, 223.6, and 224.5 kcal/mol, respectively (relative to the proton affinity of NH3 = 204.0 kcal/mol). The proton affinity measured for phenylalanine is in excellent agreement with reported literature values. The photoelectron spectra of these three molecules are also presented and analyzed. Assignments of bands to specific ionization processes are aided by comparison with model compounds such as methyl-substituted amines and 2-phenylethylamines. These data are employed to examine the correlation of adiabatic nitrogen lone pair ionization energies with gas-phase proton affinities for phenylalanine, N-methylphenylalanine, and N,N-dimethylphenylalanine in comparison to correlations for other amino acids and selected aliphatic amines. Although amine nitrogen methylation increases the potential for localizing charge at the amine terminus of protonated peptides by increasing the gas-phase proton affinity, the present study establishes that the increase is not sufficient to compete with protonation of some of the more basic side chains in peptides.
• Lichtenberger, D. L., Subramanian, L., Bunz, U., & Peter, K. (1994). Sigma-pi interactions in non-conjugated polyalkynes: A photoelectron spectroscopic study. Journal of the Chemical Society, Perkin Transactions 2, 1351-1357.
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  Abstract: Through-space and through-bond interactions between π-orbitals in the molecules hepta-1,6-diyne (A), 3,3-diethylpenta-1,4-diyne (B), 4,4-diprop-2-ynylhepta-1,6-diyne (C), 2,2-di(bromomethyl)-1,3-dibromopropane (D) and 4,4-diethynylhepta-1,6-diyne (E) have been studied using gas-phase HeI photoelectron spectroscopy. The assignments of the photoelectron bands are discussed in relation to the results of extended Hückel calculations. Mixing of the π orbitals with the σ bond framework of the molecules is revealed by broadened band profiles in the π ionization region. Detailed examination of the first ionization of A suggests that one conformation is predominant under the conditions of the experiment. The terminal π orbitals are separated by too great a distance for through-space interaction, so the spread of the ionization band is entirely from through-bond interactions. The low-energy ionizations of B correspond to the in-plane and out-of-plane symmetric and antisymmetric combinations of the four terminal π orbitals, each of which has a different interaction with the C-H and C-C bonds of the central carbon atom. The spectra of C and D are very similar to each other, even though C has only alkyne substituents and D has only bromine substituents. The spectrum of E is a complicated mix of some of the features of A and some of the features of B. Extended Hückel calculations help clarify the number of orbitals in this region and the nature of the orbital interactions.
• Lichtenberger, D. L., Wright, L. L., Gruhn, N. E., & Rempe, M. E. (1994). Electronic structure of exohedral interactions between C₆₀ and transition metals. Journal of Organometallic Chemistry, 478(1-2), 213-221.
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  Abstract: The electron distribution and orbital interactions of C60 with metals coordinated at different sites on the outside of the fullerene are evaluated. These sites include the position of a metal atom directly above a carbon atom (η1 site), the metal atom centered above two carbons of a pentagon or above two carbons between two pentagons (both η2 sites), the metal atom centered above a pentagon (η5 site), and the metal atom centered above a hexagon (η6 site). The frontier orbitals of C60 are illustrated first with three-dimensional orbital contour plots. A palladium atom is then used to probe the bonding at the different sites on the C60 surface. The results with Pd0 are compared to our earlier study with the harder Ag+ ion in order to examine the effects of metal electron richness and size. In addition, these results are compared with the bonding to more traditional ligands that represent the hapticity of these sites, such as methyl (η1), ethylene (η2), cyclopentadienyl (η5), and benzene (η6). The strength of the metal-C60 interaction and the amount of charge delocalized from the metal to C60 is sensitive to the site of coordination, the electron richness of the metal, and distortions in the geometry of C60. As discussed in our previous work, the frontier orbitals of C60 are well-suited for synergistic bonding of a metal atom to a carbon-carbon pair in an alkene-like fashion, in which the HOMO of C60 donates carbon-carbon π bonding electron density to the metal, and the LUMO of C60 accepts electron density from the metal into a carbon-carbon π* antibonding orbital. Although the HOMO and LUMO of C60 describe the basic interaction, many frontier orbitals are involved. The site above the CC bond between two pentagons is favored over the site above the CC bond within a pentagon, and the interaction above the other sites is indicated to be net repulsive by these calculations. The differentiation between these sites increases with the electron richness of the metal center. The bonding of the metal to C60 is generally weaker than to the small ligands, except for very electron rich metal centers where the bonding to the η2 site between pentagons apparently becomes stronger than the bonding to ethylene. © 1994.
• Ryan, M. F., Richardson, D. E., Lichtenberger, D. L., & Gruhn, N. E. (1994). Gas-phase ionization energetics, electron-transfer kinetics, and ion solvation thermochemistry of decamethylmetallocenes, chromocene, and cobaltocene. Organometallics, 13(4), 1190-1199.
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  Abstract: The gas-phase free energies of ionization, ΔGi° for Cp*2Mn, Cp±2Fe, Cp*2Ni, Cp±2Os, Cp2Cr, and Cp2Co (Cp = η5-cyclopentadienyl, Cp* = η5-pentamethylcyclopentadienyl) have been determined by using the electron-transfer equilibrium (ETE) technique and Fourier transform ion cyclotron resonance mass spectrometry. The high-resolution valence photoelectron spectra of bis(benzene)chromium(0), Bz2Cr, Cp*Os, and Cp*2Ru have also been measured. Most of the ΔGi° values are referenced to the estimated ΔGi° value of Bz2Cr, for which the narrow first ionization band at 5.473 ± 0.005 eV is assigned as the adiabatic ionization potential. The ΔSi° for ionization of Bz2Cr is assumed to be equal to the electronic entropy change, ΔSelec° (= 1.4 cal mol-1 K-1), and the difference between the integrated heat capacities for Bz2Cr and Bz2Cr+ is also assumed to be negligible near room temperature (ΔHi,0° ≈ ΔHi,350°), leading to ΔGi° (Bz2-Cr) = 125.6 ± 1.0 kcal mol-1. Through the use of thermochemical cycles, estimates are given for the average heterolytic and homolytic M-Cp bond disruption enthalpies of Cp2Cr+/0 and Cp2Co+/0. Cyclic voltammetry experiments (CH3CN/0.1 M Bu4NPF6) for the decamethylmetallocenes, including Cp*2Ru, were performed in order to determine differential solvation energies, ΔΔGsolv°, for the +/0 redox couples. Generally, ΔΔGsolv° values for the decamethyl derivatives are in the range -21 to -29 (±4) kcal mol-1. Electron-transfer kinetics for several metallocene couples were measured from the approach to equilibrium in the ETE experiments, and couples that involved Cp±2M compounds were observed to have rate constants less than 10% of the Langevin collision frequency when the free energy change was in the range 0 to -6 kcal mol-1.
• Lichtenberger, D. L., Hoppe, M. L., Subramanian, L., Kober, E. M., Hughes, R. P., Hubbard, J. L., & Tucker, D. S. (1993). Electron distribution and bonding in η³-cyclopropenyl-metal complexes. Organometallics, 12(6), 2025-2031.
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  Abstract: The synthesis of (η3-C3R3)Ir(CO)3 and He I and He II photoelectron spectra of (η3-C3R3)-Co(CO)3, (η3-C3R3)Ir(CO)3, and (η3-C3R3)Fe(CO)2(NO) (where R = tert-butyl) are reported. The shifts and splittings in ionization energies with the metal and ligand perturbations in this series, the changes in ionization peak areas as a function of the excitation energy, and Fenske-Hall molecular orbital calculations assist in the assignment and interpretation of the spectra. The cobalt complex reveals three peaks in the low ionization energy region that are primarily metal-based, consistent with the two states of e symmetry and one state of a1 symmetry from the five d orbitale of a formally d10 metal in C3v symmetry. The spectrum of the iridium complex has five peaks in this region due to large spin-orbit coupling that splits the e symmetry ionizations. The doubly degenerate peaks of the cobalt complex are also expected to be split in the photoelectron spectrum of the iron-nitrosyl complex due to the lowering to Cs symmetry. Only four distinct peaks are seen for the iron-nitrosyl complex, with two ionization bands at higher energy being merged in a broad envelope. The ionization cross-sections indicate that the η3-bound C3R3 ligand in these complexes is best described formally as a cation with a large amount of mixing and backbonding from the metal dπ orbitals to the eπ* orbitale of the cyclopropenyl ring. This is compared to the NO+ ligand. © 1993 American Chemical Society.
• Lichtenberger, D. L., Renshaw, S. K., & Bullock, R. M. (1993). Metal-acetylide bonding in (η⁵-C₅H₅)Fe(CO)₂C≡CR compounds. Measures of metal-dπ-acetylide-π interactions from photoelectron spectroscopy. Journal of the American Chemical Society, 115(8), 3276-3285.
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  Abstract: Gas-phase He I and He II photoelectron spectroscopy is used to experimentally determine the bonding interactions of η1-acetylide ligands in (η5-C5H5)Fe(CO)2C≡CR compounds (R = H, tBu, or phenyl). The spectra show a large amount of interaction between the metal dπ orbitals and the acetylide π orbitals. Evidence for this is obtained from the splitting of metal-based ionization bands, from the shifts in Cp-based and acetylide-based ionizations, from the changes in ionization cross sections between the He I and He II spectra, and from vibrational fine structure in the metal-based ionizations. The data indicate that the predominant π interactions between the acetylide ligands and the metal are filled/filled interactions between the occupied acetylide π bonds and the occupied metal dπ orbitals. The electronic interactions of the C≡CR ligands with the metal are compared with those of CH3 (a primiarly σ donor ligand), halides (π donor ligands), and C≡N (a weak π* acceptor ligand), and are very similar to the interactions of the chloro ligand. Metal-to-acetylide-π* back-bonding is extremely small in the acetylide compounds. Varying the acetylide substituent causes significant changes in the σ and π donor properties of the C≡CR ligand. Compared to C≡CH, the C≡CtBu ligand is a stronger ódonor ligand and also has a stronger filled/filled interaction between the metal dπ and acetylide π orbitals. The electronic mixing with the C≡CPh ligand is even more extensive, since the metal dπ orbitals and the C≡C π bonds are further mixed with the phenyl ring π orbitals. The nature of these π interactions helps to explain the observed reactivity of electrophiles with MLnC≡CR compounds and the observed electronic communication along the metal-carbon-carbon atom chain in transition metal-acetylide compounds.
• Lichtenberger, D. L., Renshaw, S. K., Wong, A., & Tagge, C. D. (1993). Investigation of metal-dπ-butadiynyl-π interactions in (η⁵-C₅H₅)(CO) ₂FeC≡CC≡CH using photoelectron spectroscopy. Organometallics, 12(9), 3522-3526.
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  Abstract: The electronic structure of (η5-C5H5)(CO) 2FeC≡CC≡CH (1) is investigated with gas-phase He I and He II photoelectron spectroscopy (PES). A central issue is the electronic communication from the metal through the poly-yne chain. The spectra show distinct ionization bands (ionizations) that originate from the metal d6 orbitals, the occupied butadiynyl π orbitals, the cyclopentadienyl1″ orbitals, and the iron-butadiynyl σ bond. Metal-dπ-butadiynyl-π electronic interactions are indicated by the splitting pattern of the iron-based ionizations and from changes in ionization cross sections from He I to He II excitation, which indicate the predominant character of ionizations. The C≡CC≡CH (butadiynyl) ligand is best described as a net π donor ligand, and in this case the occupied butadiynyl π orbitals interact with occupied metal dπ orbitals in filled/filled type interactions. The mixing between occupied metal dπ and butadiynyl π orbitals is extensive, and the π system of 1 can be thought of as a five-centered poly-yne. Results from Fenske-Hall and extended Hückel calculations agree with the information obtained from PES and also give additional information about orbital coefficients and charges of the C≡CC≡CH π system. The spectrum of 1 is also compared to that of (η5-C5H5)(CO)2FeC≡CH, and the separate σ and π bonding components of the C≡CH and C≡CC≡CH ligands are evaluated. © 1993 American Chemical Society.
• Lichtenberger, D. L., Wright, L. L., Gruhn, N. E., & Rempe, M. E. (1993). Electronic structure and bonding of C₆₀ to metals. Synthetic Metals, 59(3), 353-367.
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  Abstract: The electron distribution and orbital interactions of C60 with metals coordinated at different sites on the outside of the fullerene are evaluated with the Fenske-Hall molecular orbital method. The characters and nodal properties of the frontier orbitals of C60 are first evaluated in terms of basis transformations to the C2 units that join the pentagons and to the C5 units of the pentagons in the C60 molecule. The highest occupied molecular orbital (HOMO, hu symmetry) of C60 is largely π bonding between the carbon atom pairs that join adjacent pentagons. The lowest unoccupied molecular orbital (LUMO, t1u symmetry) is predominantly π antibonding between these carbon atom pairs. These orbital characters and energies are well situated for synergistic bonding of a metal atom to the carbon-carbon pair between the pentagons, in which the HOMO of C60 donates σ electron density to the metal, and the LUMO of C60 accepts π electron density from the metal. The electron donation and acceptance between the individual molecular orbitals of the C60 molecule and the orbitals of a metal at different possible bonding sites of C60 are probed with a Ag+ ion. It is found that the bonding is favored at the site between the pentagons and that many different orbitals of C60 are involved in the interaction. The net bonding of Ag+ to C60 is weaker than to ethylene. Calculations are also carried out on the organometallic complexes C60Pt(PH3)2 and (C2H4)Pt(PH3)2. The net bonding of ethylene and C60 to platinum is found to be very similar in these cases. A significant difference in this case is that the net negative charge on C60 is more delocalized in the carbon cluster in contrast to the localized charge on ethylene. © 1993.
• Campbell, S., Beauchamp, J. L., Rempe, M., & Lichtenberger, D. L. (1992). Correlations of lone pair ionization energies with proton affinities of amino acids and related compounds. Site specificity of protonation. International Journal of Mass Spectrometry and Ion Processes, 117(C), 83-99.
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  Abstract: The gas phase proton affinities of amino acids are compared with their adiabatic ionization energies obtained from photoelectron spectra. Using primary aliphatic amines as reference species, a linear correlation is found between the proton affinities and the adiabatic nitrogen lone pair ionization energies for those amino acids which protonate on the amine group, even in cases where the nitrogen lone pair is not the highest occupied molecular orbital. Many of the amino acids fit the correlation well, which confirms the prediction of amine protonation from earlier studies and also corroborates the assignment of the bands in the complex photoelectron spectra of these species. Proline and safcosine, amino acids with a secondary nitrogen, deviate from this correlation and instead fit a correlation using secondary aliphatic amines as reference species. Deviations from the correlation exist for molecules, such as lysine, methionine and tryptophan, which contain an intramolecular hydrogen bond between the basic side-chain and the amine site. The gas phase proton affinities of these species are larger than predicted by the correlation. Deviations from the correlation are also predicted for very basic amino acids, such as histidine and arginine, which protonate preferentially on the side-chain instead of the amine group. © 1992.
• Howells, S., Chen, T., Gallagher, M., Sarid, D., Lichtenberger, D. L., Wright, L. L., Ray, C. D., Huffman, D. R., & Lamb, L. D. (1992). High resolution images of single C₆₀ molecules on gold (111) using scanning tunneling microscopy. Surface Science, 274(1), 141-146.
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  Abstract: The electronic interactions of fullerene molecules with metals, with other molecules, and with themselves are important to the chemical and conductive properties of these materials. We demonstrate high resolution scanning tunneling microscopy images of C60 molecules condensed on epitaxial gold (111) films on mica, in which the C60 molecules are isolated from each other. The C60 molecules were locked in position to the gold substrate by an ordered layer of methyl isobutyl ketone. The images of the C60 molecules exhibit intramolecular contrast indicating a significant electronic interaction with the gold substrate. Current versus voltage measurements show that both the C60 and the thin film of methyl isobutyl ketone have conductances comparable to that of the gold substrate. © 1992.
• Lichtenberger, D. L., & Jatcko, M. E. (1992). Electronic structure of monodentate-coordinated diphosphine complexes. Photoelectron spectra of Mo(CO)₅(P(CH₃)₂CH₂P(CH ₃)₂) and Mo(CO)₅(P(CH₃)₂CH₂CH ₂P(CH₃)₂). Inorganic Chemistry, 31(3), 451-455.
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  Abstract: Photoelectron spectroscopy is used to study the electronic structure of molybdenum carbonyl complexes that contain diphosphine ligands bound to the metal through only one of the two phosphorus atoms. This represents the first examination of the relative bonding capabilities of diphosphine ligands in the absence of chelating geometries, which is important for understanding many chelate effects. Photoelectron spectra are reported for Mo(CO)5DMPE and Mo(CO)5DMPM and compared to the spectra of Mo(CO)5PMe3 and the corresponding free phosphine and diphosphine ligands (PMe3 is trimethylphosphine, DMPE is 1,2-bis-(dimethylphosphino)ethane, and DMPM is bis(dimethylphosphino)methane). The energy splittings between the d6 metal-based ionizations of these complexes indicate that the π-back-bonding ability is the same for each of these phosphine ligands and is relatively small, about 25% that of carbon monoxide. The metal-based ionizations shift only slightly to lower binding energy from the PMe3 to the DMPE to the DMPM complex (total shift = 0.10 eV) due to a slightly increasing negative charge potential at the metal along this series. This would normally be interpreted as slightly increasing σ-donor strength in the order PMe3 < DMPE < DMPM. However, the difference between the ionization energy of the coordinated lone pair (CLP) of the phosphine and the ionization energy of the lone pair of the free ligand indicates an opposite trend in σ-donor strength with PMe3 (1.28 eV) > DMPE (1.27 eV) > DMPM (1.23 eV). The shift of the uncoordinated phosphine lone-pair ionization (ULP) of the monocoordinated diphosphine complexes, which is affected primarily by charge potential effects, reveals that the important factor is a transfer of negative charge from the uncoordinated end of the phosphine through the alkyl linkage to the coordinated phosphine. This transfer is more important for the DMPM ligand because of the shorter alkyl chain between the phosphorus atoms. Aside from these subtle details of charge distribution, the primary conclusion is that the diphosphine ligands, DMPE and DMPM, have σ-donor and π-acceptor strengths extremely similar to those of PMe3. © 1992 American Chemical Society.
• Lichtenberger, D. L., Ray, C. D., Stepniak, F., Chen, Y., & Weaver, J. H. (1992). The electronic nature of the metal-metal quadruple bond: Variable photon energy photoelectron spectroscopy of Mo₂(O₂CCH₃)₄. Journal of the American Chemical Society, 114(26), 10492-10497.
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  Abstract: Variable-energy photoelectron spectroscopy of thin film Mo2(O2CCH3)4 on a GaAs substrate in ultrahigh vacuum is used to examine the valence ionizations of the quadruple metal-metal bond. The changes in photoionization cross sections with photon energy over a range of 40 to 90 eV are examined. The metal-based σ, π, and δ ionizations of the quadruple bond are strongly enhanced relative to the acetate-based ionizations in the region of photon energies from 40 to 50 eV. This is consistent with a molybdenum 4p to 4d resonance and super-Coster-Kronig Auger enhancement of the metal-based ionizations. The extent of resonance enhancement is related to the amount of Mo 4d character associated with the ionization. The π ionization of the Mo-Mo quadruple bond has the largest contribution from the Mo 4d orbitals. The δ and σ ionizations of the metal-metal bond contain smaller amounts of Mo 4d character, and some Mo 4d character is observed in the acetate-based ionizations. This is explained in terms of significant overlap and mixing of the δ and σ components with the ligand orbitals. Also, in the case of the σ component, there is a possible filled/filled interaction between the Mo 4dz2 orbital on one metal and the Mo 4pz orbital on the adjoining metal. These interactions have significant impact on the properties of these complexes.
• Lichtenberger, D. L., Rempe, M. E., & Gogosha, S. B. (1992). The He I valence photoelectron spectrum ofC₇₀ in the gas phase. Chemical Physics Letters, 198(5), 454-460.
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  Abstract: The high-resolution He I photoelectron spectrum of C70 in the gas phase is reported and compared with that of C60 and theoretical calculations. Similar to C60, the C70 spectrum shows two separate valence ionization band envelopes in the low energy region, but there are more separate ion states visible within the two envelopes. The first envelope of overlapping ionizations spans from about 7.3 to 8.8 eV, and the second from about 8.9 to 10.3 eV. The greater complexity of the C70 spectrum compared to that of C60 is expected due to the lower symmetry of C70 and the ten additional π electrons from the additional carbon atoms. The first vertical ionization energy is 7.47 ± 0.02 eV, which is destabilized 0.17 eV from that of C60. © 1992.
• Sarid, D., Chen, T., Howells, S., Gallagher, M., Yi, L., Lichtenberger, D. L., Nebesney, K. W., Ray, C. D., Huffman, D. R., & Lamb, L. D. (1992). Buckyball-substrate interactions probed by STM and AFM. Ultramicroscopy, 42-44(Part A), 610-615.
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  Abstract: Submonolayers of buckyball molecules (C60) on a gold substrate, deposited under ultrahigh-vacuum (UHV) conditions, have been imaged in air using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM images show intramolecular contrasts within most individual C60 molecules. AFM images of the C60 molecules, often grouped in a hexagonal arrangement, show no atomic-scale features. A possible explanation for the presence or absence of the intramolecular contrasts in the images obtained with the STM and AFM, respectively, is given by a molecular orbital calculation.
• Herrmann, W. A., Kiprof, P., Rypdal, K., Tremmel, J., Blom, R., Alberto, R., Behm, J., Albach, R. W., Bock, H., Solouki, B., Mink, J., Lichtenberger, D., & Gruhn, N. E. (1991). Multiple bonds between main-group elements and transition metal. 86.¹ methyltrioxorhenium(VII) and trioxo(η⁵-pentamethylcyclopentadienyl)rhenium(VII): Structures, spectroscopy, and electrochemistry. Journal of the American Chemical Society, 113(17), 6527-6537.
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  Abstract: Two key compounds of organometal oxides, methyltrioxorhenium(VII) (1) and trioxo(η5-pentamethylcyclopentadienyl)rhenium(VII) (2), have been structurally characterized by means of electron diffraction techniques, showing that the ReO3 fragments of these compounds have trigonal-pyramidal structures in the gas phase. The rhenium-carton distance of the 14e complex 1 amounts to 206.0 (9) pm, which is the shortest Re-C(sp3) bond so far recorded. The pentamethylcyclopentadienyl derivative 2 has the longest known rhenium-carbon bond (240.5 (6) pm) due to the size of this particular π-bonded ligand and the σ/π-donor properties of the oxo ligands ("trans influence"). Infrared and Raman spectra show a much higher triple-bond contribution in the rhenium-oxygen bonds of 1 (force constant κ = 8.16 mdyn/Å) compared with 2(κ = 6.99 mdyn/Å). The π-donor qualities of the ring ligand of 2 are considered the major effect to reduce the rhenium-oxygen order of this 18e compound since the σ-aryl complex (σ-C6H2Me3)ReO3 (3) has a force constant of κ= 8.08 mdyn/Å, According to cyclovoltammetric data, the methyl derivative 1 is more easily reduced (Epc = -0.84 V vs Ag/AgCl, THF, 20 °C) than the half-sandwich congener 2 (Epc = -1.72 V), again reflecting the electronic situation of the two compounds (14e vs 18e, respectively). The first vertical PE ionization energies of 1 and 2, 11.8 and 8.6 eV, differ by 3.2 eV due to their different radical-cation ground states, X̄(a2n0) and X̄(e,πCp*). The equivalent oxygen lone pair type ionization of 2, IEv2(a2,n0) = 9.9 eV, is lower by 1.9 eV and gives proof of the electron donation from the η5-bonded π-ligand C5Me5. In addition, the PE spectrum of trioxo(η1-mesityl)rhenium(VII) (3) has been recorded: Its first ionization energy of 9.00 eV exceeds the corresponding one of mesitylene by 0.6 eV, thus demonstrating the considerable acceptor effect of the ReO3 substituent group toward an η1-bonded π ligand. The high electric dipole moment of 2 (μ = 6.2 D; benzene, 25 °C) appears reasonable in light of the high polarizability of the C5Me5-Re bond (approximately 4 D); the ReO3 unit has a dipole increment of ca. 2.2 D in 1 and 2. N MR and PE spectra clearly show that the ReO3 functionality is a strong electron-withdrawing substituent, stereoelectronically comparable with the SO3H substituent in organic compounds.
• Lichtenberger, D. L., & Rai-Chaudhuri, A. (1991). Electronic structure of transition metal-silicon bonds. Valence photoelectron spectra of (η⁵-C₅H₅)Fe(CO)₂L complexes (L = SiCl₃, Si(CH₃)₃). Journal of the American Chemical Society, 113(8), 2923-2930.
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  Abstract: The HeI and HeII photoelectron spectra of (η5-C5H5)Fe(CO)2SiCl3 and (η5-C5H5)Fe(CO)2Si(CH 3)3 have been obtained in order to examine the bonding of silyl ligands to transition metals. The chemistry of both complexes has been studied previously in relation to models for catalytic intermediates in hydrosilation reactions. The nature of the metal-silicon σ bond and the possibility of π back-bonding from the metal to empty silicon d orbitals are of particular interest to the understanding of this chemistry. Previous observations of unusually short Fe-Si bond lengths in this class of complexes have led to the proposal that silane ligands are good π-acceptors. However, the splitting pattern of the "t2g-based" metal ionizations of (η5-C5H5)-Fe(CO)2Si(CH 3)3 shows no evidence of stabilization of metal d orbitals as would occur with π-back-bonding to the silane. The splitting is essentially the same as observed in the spectrum of (η5-C5H5)Fe(CO)2H, where the hydride clearly has no π-acceptor capability. This same splitting is also observed in the spectrum of (η5-C5H5)Fe(CO)2CH3. A reduced splitting is observed in the spectrum of (η5-C5H5)Fe(CO)2SiCl 3. Evaluation of these ionizations shows that SiCl3 is a better π-acceptor than CN and is about half as effective as CO at π stabilization of the metal ionizations. The π-back-bonding to SiCl3 probably involves significant portions of the Si-Cl σ* orbitals. The short Fe-Si bond in the SiCl3 complex is a combination of π-back-bonding and ionic bonding contributions. The ionizations of (η5-C5H5)Fe(CO)2SiCl3 are also compared with the ionizations of the "isoelectronic" (η5-C5H5)Mn(CO)2-HSiCl 3 complex. The manganese complex is related to the iron complex by a "hydride shift" from the iron nucleus, and the ionization shifts help to reveal the extent of oxidative addition of the Si-H bond to the manganese center. The strength of the Mn-SiCl3 bond contributes to the nearly complete oxidative addition of the Si-H bond to the metal. The implications of these results to the proposed mechanisms of hydrosilation reactions are discussed. The ionization trends in these complexes favor olefin insertion into the metal-silyl bond in the case of trialkylsilyl complexes.
• Lichtenberger, D. L., Hogan, R. H., Healy, M. D., & Barron, A. R. (1991). Electronic structure and bonding in four-coordinate organometallic complexes of aluminum. Valence photoelectron spectra of (CH₃)₃Al(pyridine) and (CH₃)₂(BHT)Al(pyridine) (BHT = 2,6-di-tert-butyl-4-methylphenoxide). Organometallics, 10(3), 609-614.
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  Abstract: The He I valence photoelectron spectra of the Lewis acid-base adducts, Me3Al(Py) and Me2(BHT)Al(py) (BHT = 2,6-di-tert-butyl-4-methylphenoxide, py = pyridine, Me = methyl) have been obtained in order to characterize the electronic structure and bonding in four-coordinate organometallic complexes of aluminum. In the photoelectron spectrum of the BHT ligand coordinated to aluminum in Me2(BHT)Al(py), the phenyl π a2 ionization of the phenoxide ligand (Phπ a2) retains the vibrational structure observed in the free alcohol (BHT-H), but the individual vibrational components are lost in the ionization, which corresponds most closely with the Phπ b1. The loss of vibrational fine structure associated with ionization from the Phπ b1 orbital in the coordinated phenoxide is one evidence that the phenoxide is involved in a π interaction with the Me2Al(py) portion of the molecule. The similarity of the splits of the Phπ a2 and b1 ionizations in Me2(BHT)Al(PMe3) and Me2(BHT)Al(py) shows that the extent of the π interaction of the BHT ligand with the Me2Al(PMe3) and Me2Al(py) portions of the molecules is about the same. The Phπ a2 and b1 ionizations of the BHT ligand experience a destabilizing shift on proceeding from Me2(BHT)Al(py) to Me2(BHT)Al(PMe3). This increase in negative charge potential at the metal center when the trialkylphosphine replaces the pyridine follows from the expected better σ-donor ability of the trialkylphosphine and the potentially better π-acceptor ability of the pyridine ligand. The pyridine ionizations show several interesting trends. First, the pyridine π ionizations in Me3Al(py) are destabilized in comparison to the same ionizations of free pyridine, showing that the pyridine experiences a net negative charge potential when it is coordinated to Me3Al. Since the aluminum is in its highest formal oxidation state, the net negative charge potential must involve the Al-Me σ bonds and/or π donation from the methyl C-H σ bonds. Second, on proceeding from Me3Al(Py) to Me2(BHT)Al(py), the pyridine π ionizations show the expected stabilization from the more positive potential at the aluminum center that results from replacing a methyl group with the more electronegative alkoxide. However, the coordinated nitrogen lone-pair ionization shows no significant shift. This again traces to overlap interaction with the π-donor orbital of the alkoxide. These results suggest that π-symmetry interactions with ligands from the first row of the periodic table may be a prevalent aspect of the chemistry of four-coordinate aluminum complexes. © 1991 American Chemical Society.
• Lichtenberger, D. L., Nebesny, K. W., Ray, C. D., Huffman, D. R., & Lamb, L. D. (1991). Valence and core photoelectron spectroscopy of C₆₀, buckminsterfullerene. Chemical Physics Letters, 176(2), 203-208.
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  Abstract: The MgKα X-ray (XPS) and He(I)/He(II) ultraviolet (UPS) photoelectron spectra of several thin films of C60 (one to three monolayers) prepared by vapor deposition on gold are reported. The core XPS spectrum shows a single narrow carbon 1s ionization at 285.1 eV. The valence UPS spectra show very sharp valence ionization bands, comparable to gas phase spectra, with wide separations between the lowest ionization energy features. The ionizations are consistent with theoretical calculations based on the highly symmetric truncated icosahedral structure. The first vertical ionization energy relative to the vacuum level is estimated to be 7.6 ± 0.2 eV from these solid state measurements. © 1991.
• Lichtenberger, D. L., Rai-Chaudhuri, A., Seidel, M. J., Gladysz, J. A., Agbossou, S. K., Igau, A., & Winter, C. H. (1991). Delocalized electronic interactions in chiral cyclopentadienylrhenium halide complexes. Valence photoelectron spectra of CpRe(NO)(L)X (Cp = η⁵-C₅H₅, η⁵-C₅(CH₃)₅; L = CO, P(C₆H₅)₃; X = Cl, Br, I). Organometallics, 10(5), 1355-1364.
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  Abstract: The electron energies and distributions of the title complexes are investigated by valence photoelectron spectroscopy. Assignment and characterization of the ionization bands are assisted by the trends in He I and He II cross sections for ionization intensities, as well as by the widths and vibrational progressions of certain ionization bands. The shifts of the ionizations, which are key to revealing the nature of the electronic structure interactions in a system, are caused by halogen substitutions on the metal, methyl substitutions on the cyclopertadienyl rings, and phosphine substitutions for the carbonyls. The first three ionizations, corresponding to the three occupied d orbitals of the d6 Re(I) metal center, are widely separated. This is because each of the three metal d orbitals have very different π interactions with the three different CO, NO, and halogen ligands. The ionizations indicate extensive metal-halogen mixing and a very delocalized electronic structure throughout the metal-ligand system. The first two ionizations correspond to the metal-halogen π* interaction, which results from the filled-filled interaction, with the first ionization including back-bonding to the carbonyl and the second ionization including back-bonding to the nitrosyl. The third ionization involves back-bonding to both the carbonyl and the nitrosyl. The first two ionizations of the iodide complexes show short vibrational progressions corresponding to the CO and NO stretches. The difference in stabilization of the metal levels by metal-carbonyl and metal-nitrosyl back-bonding is determined from the splitting between the first two ionization bands. The M-X π* orbitals have increasing halogen character from the chloride complex to the iodide complex, but unlike for the previously studied Re(CO)5X complexes, where the first ionization shifted from predominantly metal to predominantly halogen in proceeding from X = Cl to X = I, in this case the first three ionizations remain predominantly metal throughout. There is indication in one case of further delocalized interaction of the M-X π electrons with Cp-based electrons. The extent of delocalized and fluid electron density in these systems is in contrast to other systems like (η5-C5H5)Fe(CO)2X and (CO)5ReX. © 1991 American Chemical Society.
• Lichtenberger, D. L., Renshaw, S. K., Basolo, F., & Cheong, M. (1991). Photoelectron spectroscopy and rates of CO substitution of (η⁵-C₅H₄X)Rh(CO)₂ compounds. Separation of σ and π Ring substituent effects. Organometallics, 10(1), 148-156.
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  Abstract: The He I photoelectron spectra for a series of monosubstituted (η5-C5H4X)Rh(CO)2 compounds are reported (where X = NO2, CF3, Cl, H, CH3, NMe2) and compared to the rates of carbonyl substitution reactions. The carbonyl substitution by phosphine follows an associative mechanism, and the rates are generally inhibited by greater electron richness at the metal center in these compounds. However, the rates for certain substitutions, particularly when X is Cl or NMe2, are faster than indicated by the inductive characteristics of these groups. The photoelectron spectra of the (η5-C5H4X)Rh(CO)2 compounds illustrate the effects of X on the electronic structure and rates of substitution. Pronounced shifts are seen in the cyclopentadienyl π and metal d valence ionizations as the X group is varied. The shifts of most of the valence ionizations closely follow the inductive capabilities of the X substituents, as also indicated by correlations with Hammett σ values and the carbonyl stretching frequencies of the compounds. Certain ionizations are also affected by orbitals of the X group that have π symmetry with respect to the cyclopentadienyl ring. Thus, the ionization energy shifts provide a relative measure of the inductive and resonance (π) interaction between X and the compound. The rates of CO substitution correlate with the ionizations when the shifts due to both the inductive and resonance (π) effects are taken into account. These results suggest that the "slipped ring" intermediate (η3-C5H4X)Rh(CO)2PPh3 is stabilized through π delocalization on the cyclopentadienyl ring, thus enhancing the rates of substitution. © 1991 American Chemical Society.
• Chang, C. S., Rai-Chaudhuri, A., Lichtenberger, D. L., & Enemark, J. H. (1990). He I valence photoelectron spectra of oxomolybdenum(V) complexes containing diolato or alkoxide ligands. Polyhedron, 9(15-16), 1965-1973.
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  Abstract: The He I valence photoelectron spectra (PES) are reported for several monooxo-molybdenum(V) compounds with the general formula of LMoO[O(CH2)nO] and LMoO(OR)2 (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; n = 2-4; R = Me, Et, nPr). The spectra show that the size of the metal-chelate ring in these diolato complexes has a substantial effect on the HOMO ionization. As the chelate ring size in the diolato complexes increases, the HOMO shifts (ca 0.21-0.24 eV) to lower ionization energy. The diolato complexes have their HOMO's at a higher ionization energy than analogous open chain bis-alkoxide complexes possessing the same number of carbon atoms. The unconstrained bis-alkoxide complexes show much smaller shifts ( ≤ 0.11 eV) in the ionization potential associated with the HOMO upon the addition of a CH2 unit to each alkoxide group. These gas-phase results are similar to the results observed by electrochemistry in acetonitrile solution, where both the diolato and bis-alkoxide complexes become easier to oxidize upon increasing the number of CH2 groups, as indicated by their half-wave potentials. The pπ orbital ionizations of the oxygen atoms in the alkoxide functions are also shifted (ca 0.12 eV) to lower ionization energy upon adding CH2 residues to the alkoxide chain. © 1990.
• Lichtenberger, D. L., & Copenhaver, A. S. (1990). Ionization band profile analysis in valence photoelectron spectroscopy. Journal of Electron Spectroscopy and Related Phenomena, 50(2), 335-352.
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  Abstract: Analytical descriptions of partially resolved and unresolved vibrational progressions in valence photoelectron ionization bands are discussed. The 2T2g valence ionization of Cr(CO)6 is examined as an example of a partially resolved vibrational progression, and the Jahn-Teller split 2E′ and 2E″ valence ionizations of Fe(CO)5 are examined as examples of overlapping ionizations with unresolved vibrational progressions. Asymmetric Gaussian band shapes are obtained when vibrational broadening is responsible for the overall contour of the molecular ionization. Attempts to model the valence ionizations with symmetric Gaussian peak models can lead to serious misrepresentations of the ionization bands. This is found to be particularly important for overlapping ionizations such as the Jahn-Teller split bands of Fe(CO)5, where attempts to model the contours with symmetric Gaussian peak shapes lead to physically unreasonable band positions, relative amplitudes, halfwidths and band areas. The treatment of ionizations with resolved vibrational fine structure is also addressed with the 2A1gionization of osmocene. A series of related Gaussian components are used to model the resolved vibrational progressions. The goodness of fit between the model and the data is discussed in each case. Advantages and limitations in the analytical representation of valence photoelectron data are addressed. © 1990.
• Lichtenberger, D. L., & Rai-Chaudhuri, A. (1990). Cyclopentadienyl ring methylation and its effect on Si-H bond activation in (η⁵-C₅H_5-n(CH₃) _n)Mn(CO)₂HSiH(C₆H₅)₂ (n = 0, 1, 5) complexes. Organometallics, 9(5), 1686-1690.
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  Abstract: The He I photoelectron spectra of (η5-C5H5)Mn(CO)2HSiHPh 2, (η5-O5H4CH3)Mn(CO) 2HSiHPh2, and (η5-C5(CH3)5)Mn(CO) 2HSiHPh2 (Ph is C6H5) are compared to observe the effect of cyclopentadienyl ring methylation on the extent of Si-H bond interaction with the transition metal in these complexes. © 1990 American Chemical Society.
• Lichtenberger, D. L., & Rai-Chaudhuri, A. (1990). Electronic structure control of Si-H bond activation by transition metals.2.Valence photoelectron spectra of (η⁵-C₅H ₄CH₃)Mn(CO)₂HSiPh₃,(η ⁵-C₅H₄CH₃)Mn(CO)₂ HSiHPh₂,and (η⁵-C₅H₄CH ₃). Journal of the American Chemical Society, 112(7), 2492-2497.
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  Abstract: The He I photoelectron spectra of (η5-C5H4CH3)Mn(CO) 2HSiHPh2, (η5-C5H4CH3)Mn(CO) 2HSiPh3, and (η5-C5H4CH3)Mn(CO) 2HSiFPh2 (Ph = C6H5) have been obtained in order to measure the nature and extent of Si-H bond interaction with the transition-metal center in these complexes. The principal electronic structure factors contributing to the addition of the Si-H bond to the transition metal involve the interaction of the Si-H σ and σ* orbitals with the metal. The extent of Si-H σ* interaction with the metal is obtained from the shape and splitting pattern of the metal-based ionization band. The electron distribution between the Si-H bond and the metal is indicated by the relative stabilities of the metal-based and ligand-based ionizations. It is found that the metal-based ionizations of these complexes reflect the formal d6 electron count at the metal center. Also, the small shifts of the valence ionizations reveal that the extent of electron charge density shift from the metal to the ligand is negligible. These observations show that the electronic structure of the Si-H interaction with the metal is in the initial stages of Si-H bond addition to the metal, before oxidative addition has become prevalent. The mechanism of interaction of the Si-H bond with the Mn center is predominantly through interaction of the filled Si-H σ-bonding orbital with empty metal orbitals. This contrasts with our previous photoelectron studies on (η5-C5H5)Mn(CO)2HSiCl 3, where the experimental data showed this compound to be a nearly complete oxidative addition product with formation of discrete Mn-H and Mn-Si bonds. The combination of the present study with the previous results demonstrates that complexes can be obtained that stabilize different stages of activation of the Si-H bond. These stages range from weak coordination of the Si-H bond to the metal to complete oxidative addition, depending on the substituents on the silicon atom.
• Lichtenberger, D. L., & Rai-Chaudhuri, A. (1990). Electronic structure factors of Ge-H bond activation by transition metals. Photoelectron spectra of [mn(η⁵-C₅H₅)(CO)₂(HGePh ₃)], [mn(η⁵-C₅H₄Me)(CO)₂(HGePh ₃)], and [mn(η⁵-C₅Me₅)(CO)₂(HGePh ₃)]. Journal of the Chemical Society, Dalton Transactions, 2161-2166.
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  Abstract: The He I photoelectron spectra of [Mn(η5-C5H5)(CO)2(HGePh 3)], [Mn(η5-C5H4Me)(CO)2-(HGePh 3)], and [Mn(η5-C5Me5)(CO)2(HGePh 3)] have been obtained to measure the nature and extent of Ge-H bond interaction with the transition metal centre in these complexes. The principal electronic structure factors contributing to the addition of the Ge-H bond to the transition metal involve the interaction of the Ge-H σ and σ* orbitals with the metal. The shape and splitting pattern of the metal-based ionisation band indicates the extent of Ge-H σ* interaction. The electron distribution between the Ge-H bond and the metal is indicated by the relative stabilities of the metal- and ligand-based ionisations. The electron charge-density shift from the metal to the ligand is negligible in these three complexes and the metal ionisations reflect the formal d6 electron count at the metal centre. The electronic structure of the Ge-H interaction with the metal is in the initial stages of Ge-H bond addition to the metal, before oxidative addition has become prevalent. The mechanism of interaction of the Ge-H bond with the manganese centre is predominantly through interaction of the filled Ge-H σ-bonding orbital with the empty metal orbitals. It is concluded that the magnitude of Ge-H σ interaction with the metal centre is similar to that of the corresponding Si-H σ interaction.
• Lichtenberger, D. L., & Rai-Chaudhuri, A. (1990). Electronic structure factors of Si-H bond activation by transition metals. Valence photoelectron spectra of (η⁵-C₅H₄CH₃)Mn(CO)(PMe ₃)HSiCl₃ and (η⁵-C₅H₄CH₃)Mn(CO)(PMe ₃)HSiHPh₂ (Me = CH₃, Ph = C₆H₅). Inorganic Chemistry, 29(5), 975-981.
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  Abstract: The valence photoelectron spectra of (η5-C5H4CH3)Mn(CO)(L)HSiCl 3 and (η5-C5H4CH3)Mn(CO)(L)HSiHPh 2, where L is CO or P(CH3)3, are compared to determine the effect of ligand substitution at the metal center on Si-H bond activation. Metal centers that are more electron rich may promote more complete oxidative addition of the Si-H bond to the metal. The shifts in the metal and ligand ionization energies and the relative intensities of ionizations in the He I and He II photoelectron experiments show that the metal in (η5-C5H4CH3)Mn(CO)(PMe 3)HSiCl3 is best represented by a formal oxidation state of III (d4 electron count). This indicates nearly complete oxidative addition of the Si-H bond to the metal center and results in independent Mn-H and Mn-Si bonds. In contrast, the splitting and intensity pattern of the metal-based ionizations of (η5-C5H4CH3)Mn(CO)(PMe 3)-HSiHPh2 reflect the formal d6 electron count of a metal corresponding to oxidation state I. The extent of electron charge density shift from the metal to the ligand is also small, as evidenced by the negligible shifts of these ionizations from those of the related (η5-C5H4CH3)Mn(CO) 2(PMe3) complex. These observations indicate that the electronic structure of the Si-H interaction with the metal in this complex is in the initial stages of Si-H bond addition to the metal, before oxidative addition has become prevalent. Comparison with the previously reported photoelectron spectra of (η5-C5H5)Mn(CO)2HSiCl 3 and (η5-C5H4CH3)Mn(CO) 2HSiHPh2 shows that the Si-H bond interaction with the transition metal is affected more by alkyl and halogen substitutions on silicon than by substitution of a carbonyl with typical two-electron donor ligands at the metal center. © 1990 American Chemical Society.
• Lichtenberger, D. L., Copenhaver, A. S., & Hubbard, J. L. (1990). The electronic structure of nitrosyl and carbonyl supported metal-metal interactions. The photoelectron spectra of.... Polyhedron, 9(15-16), 1783-1797.
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  Abstract: Title full: The electronic structure of nitrosyl and carbonyl supported metal-metal interactions. The photoelectron spectra of [η5-(C5H5)Fe(μ-NO)]2, [η5-(C5(CH3)5)Fe(μ-NO )]2, [η5-(C5H5)Ru(μ-NO)]2 and [η5-(C5(CH3)5)Co(μ-CO )]2. The valence photoelectron ionizations of[CpFe(μ-NO)]2, [Cp*Fe(μ-NO)]2, [CpRu(μ-NO)]2 and [Cp*Co(μ-CO)]2 (Cp = η5-C5H5 and Cp* = η5-C5(CH3)5) are examined in comparison to several theoretical calculations of the electronic structure and bonding in these complexes. The photoelectron spectra of this group of complexes, when collected together, allow identification of the eight valence metal-based ionizations. There is considerable disparity among the various calculations on the predicted order of these ionizations. The combination of Fenske-Hall calculations with experimental observations of ionization trends between isoelectronic first row complexes [Fe(NO) vs Co(CO)], between first and second row complexes (Fe vs Ru), and between complexes with ring methylation (Cp vs Cp*) allows a consistent assignment of the valence ionizations. The interplay of theory and experiment gives unique insight into the nature of bridging ligand coordination and clarifies the relative strengths of metal-ligand and metal-metal interactions, in each complex. It is found that as many metal-metal antibonding orbitals are occupied as metal-metal bonding orbitals, so that the formal metal-metal bond order is zero. The interactions are important in defining the bonding and stability of the complexes. These interactions lead to large ligand character in two of the valence ionizations. © 1990.
• Lichtenberger, D. L., Hogan, R. H., Healy, M. D., & Barron, A. R. (1990). Electronic structure and bonding in four-coordinate organometallic complexes of aluminum. Valence photoelectron spectra of BHT-H, Me₃Al(PMe₃), and Me₂(BHT)Al(PMe₃). Journal of the American Chemical Society, 112(9), 3369-3374.
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  Abstract: The He I valence photoelectron spectra of the Lewis acid-base adducts Me3Al(PMe3) and Me2(BHT)Al(PMe3) (BHT-H = 2,6-di-tert-butyl-4-methylphenol) have been obtained to characterize the electronic structure and bonding in four-coordinate organometallic complexes of aluminum. To aid in the assignment of the spectrum of Me2(BHT)Al(PMe3), the spectrum of the free alcohol, BHT-H, was also obtained. The first and second ionizations of the free BHT-H alcohol show vibrational progressions associated with the symmetric C-C phenyl ring stretching modes, consistent with the b1 and a2 π ionizations, respectively, of monosubstituted phenyl rings. In the photoelectron spectrum of BHT coordinated to aluminum in Me2(BHT)Al(PMe3), the corresponding phenoxide a2 ionization retains the vibrational structure, but the individual vibrational components are lost in the ionization that corresponds most closely with the b1. The loss of vibrational fine structure associated with ionization from the phenyl π b1 orbital in the coordinated phenoxide shows that the phenoxide is involved in a π interaction with the Me2Al(PMe3) portion of the molecule. In addition, the aluminum center in Me2(BHT)Al(PM3) feels a more negative charge potential than the aluminum center in Me3Al(PMe3), as shown by the Al-P σ ionization occurring at lower binding energy in Me3Al(PMe3). This is counter to the σ inductive effects of an alkoxide compared to an alkyl and shows that the BHT is acting as a π electron donor. The change in band shape of the Al-P σ ionization between Me3Al(PMe3) and Me2(BHT)Al(PMe3) indicates that the oxygen p π orbital of the phenoxide ligand is interacting directly with the Al-P bonding orbital. The relationship between experimental ionization potentials and bond strengths of the Al-P σ bond in Me3Al (PMe3) and Me2(BHT)Al(PMe3), consistent with π donation from the phenoxide into the predominantly Al-P σ* orbital.
• Lichtenberger, D. L., Johnston, R. L., Hinkelmann, K., Suzuki, T., & Wudl, F. (1990). Relative electron donor strengths of tetrathiafulvene derivatives: Effects of chemical substitutions and the molecular environment from a combined photoelectron and electrochemical study. Journal of the American Chemical Society, 112(9), 3302-3307.
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  Abstract: Interest in organic metals and superconductors has prompted studies of the effects of chemical substituents on the organic electron donor tetrathiafulvalene (TTF). Electron-donating substituents on TTF should lead to reduced ionization potentials and generally greater electron transfer in organic donor/acceptor compounds. However, the relative electron donor abilities may also be influenced by their molecular environment and intermolecular interactions. In order to address these questions, the valence ionization potentials of TTF and two derivatives, bis(ethylenedioxo)tetrathiafulvalene (BEDO-TTF) and bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), have been measured in the gas phase by photoelectron spectroscopy and compared with oxidation potentials from solution electrochemical measurements in a variety of solvents. The order of decreasing first ionization potentials is BEDT-TTF ≈ TTF (6.7 eV) > BEDO-TTF (6.46 eV). However, the order of solution oxidation potentials is BEDT-TTF > BEDO-TTF > TTF. The solvent dependence of these oxidation potentials scales linearly with the cation solvation power of the solvent, expressed as the difference between the first and second oxidation potentials of BEDO-TTF. Extrapolation to the zero-solvation limit (i.e., the gas phase) reproduces the order of ionization potentials determined by photoelectron spectroscopy. When predicting whether a derivative is a better electron donor than TTF, it is therefore necessary to consider the molecular environment of the donor molecule. The nature of solvation and/or interaction with the acceptor molecules may result in trends that are opposite to those based purely on ionization potentials or theoretical calculations on single isolated molecules.
• Lichtenberger, D. L., & Copenhaver, A. S. (1989). Experimental measures of the electron distribution and bonding in bis(η⁵-cyclopentadienyl) osmium from He(I) and He(II) valence photoelectron spectroscopy. The Journal of Chemical Physics, 91(2), 663-673.
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  Abstract: The He(I) and He(II) high resolution photoelectron spectra of osmocene are reported. Vibrational fine structure is observed in all the valence metal-based ionizations and in the cyclopentadienyl π ionizations which derive from the e1g and e1u symmetry combinations. Analysis of the vibrational progressions found in the metal-based ionizations provides a measure of the force constants and vibrational frequencies for the metal-ring stretch in the positive ions. The vibrational analysis for the 2E 2(5/2) and 2E2(3/2) states of the osmocene cation [derived from the spin-orbit split ionization of the metal e2g (dx2-y2, dxy) set] indicates an 0.12 Å greater metal-ring bond length in the cation compared to the neutral molecule. The sharp 2A1(1/2) ionization [correlating with removal of an electron from the metal a1g (dz2) orbital] is observed to have a much shorter vibrational progression. The adiabatic ionization is the most intense (vertical) band of the series, indicating that there is no appreciable change in metal-ring bond distance upon ionization from the nonbonding a1g (dz2) orbital to produce the 2A1(1/2) cationic state. A large spin-orbit coupling is observed in the metal ionization region and a moderate amount in the cyclopentadienyl ring π ionization region. From evaluation of the spin-orbit coupling, the ionization band which correlates primarily with the e1g combination of the cyclopentadienyl π orbitals is shown to contain about 27% metal character. The spin-orbit coupling effects are negligible in the ionization which corresponds to the e1u combination of the ring orbitals, as expected from the lack of symmetry interaction with the metal d orbitals. The ionizations of this complex also provide experimental measures of relative metal-based and carbon-based ionization cross sections with He(I) and He(II) energy sources which are compared with theoretical calculations. All of the observations indicate that covalent bonding is more prevalent in osmocene than in ferrocene. © 1989 American Institute of Physics.
• Lichtenberger, D. L., & Rai-Chaudhuri, A. (1989). Electronic structure factors of Si-H bond activation by transition metals. The valence photoelectron spectrum of (n⁵-C₅H₅)Mn(CO)₂HSiCl₃. Journal of the American Chemical Society, 111(10), 3583-3591.
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  Abstract: The valence photoelectron spectra of HSiCl3 and CpMn(CO)2HSiCl3 (Cp = n5-C5H5) have been obtained to provide a measure of the electronic structure factors that contribute to the interaction of the Si-H bond with a transition-metal center. The lowest valence ionizations of other CpMn(CO)2(ligand) complexes are metal-based and reflect the formal d6 electron count at the metal. The lowest valence ionizations in the He I photoelectron spectrum of CpMn(CO)2HSiCl3 are more stable and split over a wider energy range than has been observed previously. The chlorine lone pair based ionizations of HSiCl3 coordinated to the metal have shifted about 1 eV to lower ionization energy from those of the free HSiCl3 molecule. Both the stabilization of the metal-based ionizations and the destabilization of the ligand-based ionizations show that electron charge shift from the metal to the ligand more than compensates for the initial σ donation from the Si-H bond to the metal in CpMn(CO)2HSiCl3, as expected if the bonding has proceeded significantly toward oxidative addition. The relative intensity of the higher ionization energy component in the metal ionization region decreases with He II excitation, showing that this band is actually associated more with ligand character. This He I/He II relative intensity behavior corresponds to a formal d4 electron count for the metal, consistent with the Mn(III) oxidation state. Fenske-Hall calculations indicate that the metal d hybrid orbitals available for the Mn-Si and Mn-H bonds form an acute angle between 50° and 70°, and these directed hybrids are responsible for the close proximity of the silicon and hydrogen atoms in the complex. These results are related to the other physical and chemical properties of this complex and are contrasted with the results of our similar photoelectron studies on the interaction of a carbon-hydrogen bond with the metal center in (n3-C6H9)Mn(CO)3, where the interaction primarily involves donation of C-H σ bonding orbital electron density into the empty metal orbitals in the formation of a 3-center 2-electron bond. © 1989 American Chemical Society.
• Lichtenberger, D. L., Darsey, G. P., Kellogg, G. E., Sanner, R. D., Young Jr., V. G., & Clark, J. R. (1989). Relative strengths of early transition metal M-H and M-C bonds in substituted niobocenes and tantalocenes. Thermodynamic trends and electronic factors of olefin insertion into a metal-hydride bond. Journal of the American Chemical Society, 111(14), 5019-5028.
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  Abstract: Principles of M-C and M-H bonding interactions are examined experimentally by high-resolution valence photoelectron spectroscopy. Gas-phase He I and He II photoelectron spectra are reported for the following series of d0 and d2 bent metallocenes: Cp2MH3 (M = Nb, Ta), Cp2M(CO)L (M = Nb, Ta; L = H and M = Nb; L = CH3), and Cp2M(C2H4)L (M = Nb, Ta; L = H and M = Ta; L = C2H5) (Cp = η5-C5H5). The ligand replacements represented by this series of complexes allow stepwise comparison and assignment of each individual low-energy valence ionization band. The He I/He II spectral comparisons show that the lowest ionization energy band of the carbonyl complexes is higher in metal character than the corresponding ionization of the ethylene complexes. This trend indicates extensive electron delocalization in the metal-ethylene bonding that corresponds more accurately to a metallacyclopropane description with relatively strong metal-carbon bonds. The photoelectron data for the early transition metal hydride and alkyl species show that the metal-hydrogen bonds are slightly more stable than the metal-carbon bonds. Correlation of the ionization information for these substituted early metallocenes reveals that the stabilization of the metallacyclopropane-hydride structures, M(C2H4)H, relative to the olefin-inserted metal-alkyl structure, M(C2H5), is partly due to stabilization of the metal electron density by back-bonding to the olefin as indicated by earlier theoretical studies. However, this factor alone is not sufficient to offset the energy required to convert a carbon-hydrogen bond in M(C2H5) to a metal-hydrogen bond in M(C2H4)H. The additional important factor is the stabilization derived from conversion of the metal-alkyl σ bonding to the more favorable delocalized M-C2 σ-bonding situation in the metallacyclopropane hydride. © 1989 American Chemical Society.
• Ashby, M. T., Enemark, J. H., & Lichtenberger, D. L. (1988). Destabilizing dπ-pπ orbital interactions and the alkylation reactions of iron(II)-thiolate complexes. Inorganic Chemistry, 27(1), 191-197.
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  Abstract: For CpFe(CO)2SR (1) (R = C6H4-p-Z; Z = OMe, H, Cl, CF3, NO2) the π-type interaction between formally occupied metal d orbitals and the sulfur lone pair that is principally 3p in character has been modeled with Fenske-Hall molecular orbital calculations and experimentally investigated by gas-phase photoelectron spectroscopy. A calculation for 1 (R = H) predicts that the highest occupied molecular orbital (HOMO) is metal-sulfur antibonding and largely sulfur in character. The observed HOMO ionization energies of 1 correlate with several chemical properties, including the rate of reaction of the thiolate ligand with alkyl halides. Solvent and substituent effects on the reaction rate favor a mechanism involving nucleophilic displacement of the halide by the coordinated thiolate ligand. The nucleophilicity of the coordinated thiolate ligand of 1 is related to the metal-sulfur dπ-pπ antibonding interactions. © 1988 American Chemical Society.
• Lichtenberger, D. L., Copenhaver, A. S., Gray, H. B., Marshall, J. L., & Hopkins, M. D. (1988). Valence electronic structure of bis(pyrazolyl)-bridged iridium dicarbonyl dimers. Electronic effects of 3,5-dimethylpyrazolyl substitution on metal-metal interactions. Inorganic Chemistry, 27(24), 4488-4493.
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  Abstract: The He I valence photoelectron spectra of [Ir(μ-pyrazolyl)(CO)2]2, [Ir(μ-3-methylpyrazolyl)(CO)2]2, and [Ir(μ-3,5-dimethyl-pyrazolyl)(CO)2]2 have been obtained. These complexes may be viewed as two square-planar d8 iridium centers held together by two bridging pyrazolyl ligands to form a six-membered Ir-(N-N)2-Ir ring. The ring is in a boat conformation with the iridium atoms positioned at the bow and stern such that interaction is possible between the filled dz2 orbitals from each metal center. The 3,5-dimethylpyrazolyl complex is active as a hydrogenation catalyst while the other related complexes are not. It has been proposed previously that greater filled-filled orbital interaction between the metal centers in the dimethyl complex (as caused by the shorter metal-metal distance) accounts for the greater reactivity. It is found here that there is indeed substantial interaction between the dz2 atomic orbitals of the two iridium centers, and the ionization corresponding to the Ir-Ir antibonding dz2-dz2 interaction is the lowest energy ionization band of these complexes. This ionization is cleanly separated from the other ionizations of the complexes. Of particular interest is the broad and unusual asymmetry found in the band profile of the initial ionization, which indicates appreciable vibrational excitation associated with the shortening of the metal-metal distance upon removal of an electron from this molecular orbital. The energy of the first ionization band is very sensitive to the methyl substitutions on the pyrazolyl groups. The sensitivity of this ionization to methyl substitution (and in turn the change in reaction chemistry between these complexes) is due more to the electronic inductive effects of the methyl group substitutions than to changes in geometry and splitting of the bonding and antibonding combinations of the metal dz2 orbitals. © 1988 American Chemical Society.
• Chisholm, M. H., Clark, D. L., Huffman, J. C., G., W., Kober, E. M., Lichtenberger, D. L., & Bursten, B. E. (1987). The tungsten-tungsten triple bond. 13. Bisalkyl tetracarboxylates of dimolybdenum and ditungsten. Triple bonds between metal atoms with the valence molecular orbital description π⁴δ². Journal of the American Chemical Society, 109(22), 6796-6816.
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  Abstract: Four synthetic procedures have been developed for the preparation of compounds of formula W2R2(O2CR′)4: (1) W2R2(NMe2)4 + 4R′COOCOR′ → W2R2(O2CR′)4 + 4R′CONMe2; (2) W2R2(OR″)4 + 4R′COOH → W2R2(O2CR′)4 + 4R″OH; (3) W2R2(O2CR′)4 + 4R″COOH → W2R2(O2CR″)4 + 4R′COOH; (4) W2R6 + 4R′COOH → W2R2(O2CR′)4 + 4RH. In all but one case (R = i-Bu) the R group lacks β-hydrogen atoms, and representative combinations of R = Me, Ph, Bz (benzyl), p-tolyl, o-tolyl, np (np = neopentyl), and CH2SiMe3 with R′ = H, Me, CF3, Et, Ph, p-MeOPh, t-Bu, mesityl, and CHPh2 have been obtained by reactions in hydrocarbon or CH2Cl2 solvents at or below room temperature. Reaction 1 has been most extensively employed and has been extended to the synthesis of Mo2(np)2(O2CMe)4. Limiting factors to the generalized syntheses of M2R2(O2CR′)4 compounds involve the ease of reductive elimination from the M26+ center, which occurs more readily (i) for M = Mo than M = W and (ii) for R = a β-hydrogen-containing alkyl ligand relative to a β-hydrogen-lacking (stabilized) ligand. In the case of the latter, reductive elimination is still possible by M-C bond homolysis, which may be thermally or photochemically induced. Photolysis allows for the generalized syntheses of W2(O2CR′)4 (M-4M) compounds, including the formate, which has not previously been obtained by alternate methods. The compounds of formula M2R2(O2CR′)4 have been characterized by IR spectroscopy, mass spectroscopy, NMR studies, cyclic voltammetry, UV-visible spectroscopy, and photoelectron spectroscopy, and, in certain cases, single-crystal X-ray studies have been carried out: M = Mo, R = np, R′ = Me; M = W, (i) R = Bz, R′ = Et; (ii) R = np, R′ = Et; (iii) R = np, R′ = Ph; (iv) R = np, R′ = H; (v) R = np, R′ = Me and CF3, viz., W2(np)2(O2CMe)2(O 2CCF3)2. In the solid state all of the structurally characterized compounds have a central M2(O2C)4 paddle-wheel core, typical of M2(O2CR′)4 compounds with M-M quadruple bonds, supplemented by axially aligned M-C(alkyl/aryl) bonds. Of particular note is the fact that the W-W distances in W2R2(O2CR′)4 compounds, 2.18-2.20 Å, are essentially identical with those in the d4-d4 W2(O2CR′)4 compounds while the W-C bonds are as expected, ca. 2.17-2.21 Å. The Mo-Mo distance in Mo2(np)2(O2CMe)4, 2.13 Å, is longer by 0.04 Å than that found in Mo2(O2CMe)4 (M-4M). The M-O distances are essentially identical in the d3-d3 and d4-d4 compounds, ca. 2.08 Å. The M-M distances in the new compounds are the shortest thus far reported for d3-d3 dinuclear compounds of tungsten and molybdenum. In general, the 1H NMR studies indicate the geometry found in the solid-state is maintained in solution but, for less bulky combinations of R and R′, an alternate isomer is present; e.g., W2Me2(O2CMe)4 is spectroscopically analogous to W2Me2(O2CNEt2)4, which has equatorially aligned W-C bonds with a C2v-W2C2(O2C)4 core. For W2Bz2(O2CEt)4 a mixture of the two isomers is present in solution and the equilibrium constant is solvent dependent: the relative concentration of the C2v- to D4h-W2C2(O2C)4 isomer is ca. 1:2 in CD2Cl2 and ca. 1:8 in toluene-d8 and benzene-d6. Spectroscopic studies on the axially ligated compounds W2R2(O2CR′)4 indicate that the valence M-M MO description is π4δ2, i.e., directly analogous to that of the M2(O2CR′)4 compounds except with respect to the presence of the M-M σ bond. This conclusion is reached by a comparison of the photoelectron spectra, the UV-visible spectra, and cyclic voltammetric studies. An assignment of the 1(δ → δ*) transition is possible for both the d3-d3 and d4-d4 compounds, and the effect of conjugation of aromatic rings on the δ → π*(oco) MLCT transition is experimentally and theoretically clarified with respect to earlier studies on W2(O2CAr)4 (M-4M) compounds. In the photoelectron spectra an ionization from one of the M-C σ bonds is close in energy to the ionization from the M-M π bonds. The M-M bonds of valence MO configuration π4δ2 are remarkably short in terms of the simplistic view that occupation of M-M σ and σ* orbitals effectively leads to a cancellation of σ-bonding contributions, e.g., as in He2, 1σ2σ*2. This is not the case in M2R2(O2CR′)4 compounds because M-C and M-M σ and σ* mixing occurs such that there is residual M-M σ bonding in the d3-d3 dinuclear compounds. The magnitude of this residual M-M σ bonding is greater for M = W than M = Mo because of relativistic effects. This is the first recognition of this phenomenon for M-M multiple bonds, and a comparison with the bonding in the diatomic molecule C2 and that in HC≡CH is noted. Formally C2 has the MO configuration 1σ2σ*22σ22σ *21π4, i.e., a net double bond lacking a σ component, but s-p mixing effectively increases the bonding in 2σ and decreases the antibonding in 2σ*2. In the M2(O2CR′)4 compounds there are two significant MOs having M-M σ character because of valence s and dz2 mixing. The bonding in the R-E≡E-R units in acetylene (E = C) and in E2R2(O2CR′)4 compounds (E = Mo, W) has extensive mixing of E-R and E-E σ and σ* orbitals. The spectroscopic results are compared to a previous Xα-SW calculation [J. Am. Chem. Soc. 1985, 107, 4459]. © 1987 American Chemical Society.
• Chisholm, M. H., Heppert, J. A., Kober, E. M., & Lichtenberger, D. L. (1987). 1,3-Ditungstacyclobutadienes. 2. The synthesis of alkoxide derivatives of W₂(μ-CSiMe₃)₂(CH₂SiMe ₃)₄ and investigations of the electronic structures of the M₂(μ-CSiMe₃)₂ core as a function of dⁿ-dⁿ interactions (n = 0, M = Ta; n = 1, M = W; n = 2, M = Re). Organometallics, 6(5), 1065-1073.
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  Abstract: W2(μ-CSiMe3)2(CH2SiMe 3)4 (I) reacts with t-BuOH in alkane solvents to produce a mixture of two isomers of W2(μ-CSiMe)2(O-t-Bu)2(CH 2SiMe3)2 from which the minor isomer can be isolated by fractional crystallization. The electronic structures of several related dimetallacyclobutadiene derivatives have been investigated by UV photoelectron spectroscopy, UV/visible spectrophotometry, and MO calculations. The M-M bonding configurations for the M2(μ-CSiMe3)2(CH2SiMe 3)4 molecules have been determined as σ0δ*0, σ2δ*0, and σ2δ*2 for M = Ta, W, and Re, respectively, by EHMO and Fenske-Hall MO calculations. This result is better reconciled with the previously observed M-M distances - Nb2 > Re2 > W2 - than with the original proposal of a σ2π2 configuration for the Re derivative. On replacement of the terminal alkyl ligands of the W complex by O-i-Pr ligands, the 2au LUMO (W-W δ*) and 2b2g W-μ-C π bond are strongly destabilized by antibonding interactions with pπ orbitals on oxygen. Destabilization of the 2b2g orbital results in the appearance of a clearly defined ionization representing this orbital at 7.2 eV in the PE spectrum of W2(μ-CSiMe3)2(O-i-Pr)4. Mulliken population analyses of the 6ag orbitals of I and W2(μ-CSiMe3)2(O-i-Pr)4 correctly predict that I prefers a shorter W-W distance of 2.54 Å, while the alkoxide-substituted compound prefers the longer W-W distance of 2.62 Å. © 1987 American Chemical Society.
• Lichtenberger, D. L., & Kellogg, G. E. (1987). Experimental quantum chemistry: Photoelectron spectroscopy of organotransition-metal complexes. Accounts of Chemical Research, 20(10), 379-387.
• Lichtenberger, D. L., & Kristofzski, J. G. (1987). Intermolecular influences on M-M multiple bonds from thin-film UPS studies of group VI M₂(O₂CCH₃)₄ complexes. Journal of the American Chemical Society, 109(11), 3458-3459.
• Rye, R. R., Kelber, J. A., Kellogg, G. E., Nebesny, K. W., & Lichtenberger, D. L. (1987). Localization effects in the Auger spectra of ring nitrogen systems: Pyridine, poly(2-vinyl)pyridine, borazine, and boron nitride. The Journal of Chemical Physics, 86(8), 4375-4383.
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  Abstract: The N(KVV) Auger spectra of gas phase pyridine (C5H 5N) and borazine (B3N3H6), and of solid phase poly (2-vinyl) pyridine (PVP) and hexagonal boron nitride [(BN) x] are reported and analyzed. The data indicate two Auger "fingerprint" types of nitrogen. Ammonia (NH3) is the prototype for the first, where three of the five valence electrons are σ bonding and the other two are the lone pair. This localized electronic structure gives rise to relatively sharp features in the N(KVV) spectrum. Typical of the second fingerprint type is pyridine, where there are two σ bonding electrons, a lone pair of electrons, and one electron contributing to the delocalized π system. Theoretical nitrogen Auger transition energies and intensities are calculated for pyridine to demonstrate the general origin of the overlapping features in the relatively broad N(KVV) spectrum of this molecule. PVP fits into the second fingerprint type while borazine and boron nitride give nitrogen Auger spectra more like ammonia. Approximate calculations using the equivalent core concept are used to clarify the relationship between the ammonia, borazine, and boron nitride spectra. It is shown that in these systems the initial Auger state (core-hole) largely localizes the bonds and lone pair on the nitrogen. The Auger spectra show that it is the σ, π and nonbonding orbital characters that provide the Auger fingerprint. © 1987 American Institute of Physics.
• Ashby, M. T., Enemark, J. H., Lichtenberger, D. L., & Ortega, R. B. (1986). Acyclic polythioether complexes: Preparation and crystal structure of tricarbonyl(2,5,8-trithianonane)molybdenum(0). Inorganic Chemistry, 25(18), 3154-3157.
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  Abstract: Tricarbonyl(2,5,8-trithianonane)molybdenum(0) has been prepared and characterized by single-crystal X-ray crystallography. The compound crystallizes in the monoclinic space group P21/n with a = 7.255 (3) Å, b = 12.270 (3) Å, c = 15.229 (3) Å, β = 96.51 (2)°, V = 1346.9 (6) Å3, and Z = 4. The structure was solved by using the heavy-atom Patterson method followed by least-squares refinement using 3039 independent reflections to a final R1 value of 0.026 (R2 = 0.031). A detailed comparison is made between the structural and spectroscopic data obtained for tricarbonyl(2,5,8-trithianonane)molybdenum(0) and those previously reported for tricarbonyl(1,4,7-trithiacyclononane)molybdenum(0). This comparison suggests that the sulfur donor orbitals for tricarbonyl(2,5,8-trithianonane)molybdenum(0) are more favorably directed toward the empty metal orbitals than those for tricarbonyl(1,4,7-trithiacyclononane)molybdenum(0). © 1986 American Chemical Society.
• Lichtenberger, D. L., & Kellogg, G. E. (1986). Electronic structure factors of carbon-hydrogen bond activation. The photoelectron spectroscopy of (cyclohexenyl)manganese tricarbonyl. Journal of the American Chemical Society®, 108(10), 2560-2567.
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  Abstract: The He I and He II ionizations for (cyclohexenyl)manganese tricarbonyl, a molecule which exhibits an activated C-H bond, are reported. Comparisons are made to the electronic structures of (methylcyclopentadienyl)manganese tricarbonyl, (cyclohexadienyl)manganese tricarbonyl, and (cyclohexadiene)iron tricarbonyl. Electronic structure factors contributing to the initial activation of the C-H bond are discussed in terms of two limiting descriptions that have been presented in the literature. These descriptions are labeled σ activation, involving the donation of C-H σ bonding orbital electron density into the empty metal orbitals, and σ* activation, involving electron density flow in the opposite direction, i.e., from filled metal levels into the empty C-H σ* antibonding level. Both processes are shown to be possible based only on geomertry, symmetry, and overlap considerations. The experimental data demonstrate that the principal electronic structure mechanism for the early stages of C-H bond lengthening and interaction with the metal exhibited in (cyclohexenyl)manganese tricarbonyl is σ activation. The data are also related to variable-temperature NMR studies of this complex which show that fluxionality proceeding through a 16 e- intermediate is more favorable than that through an 18 e- intermediate with a full metal-hydrogen bond. Both bond strength and ionization energy data show that the C-H activation of (cyclohexenyl)manganese tricarbonyl stops at the agostic stage because there is no net gain in carbon-carbon or metal-carbon bonding to compensate the loss of the C-H bond. © 1986 American Chemical Society.
• Lichtenberger, D. L., & Kellogg, G. E. (1986). Erratum: Electronic structure factors of carbon-hydrogen bond activation. The photoelectron spectroscopy of (cyclohexenyl)manganese tricarbonyl (Journal of the American Chemical Society (1986) 108 (2560-2567)). Journal of the American Chemical Society, 108(22), 7134-.
• Lichtenberger, D. L., Kellogg, G. E., Kristofzski, J. G., Page, D., Turner, S., Klinger, G., & Lorenzen, J. (1986). Inexpensive and high-precision digital power supply and counting interface for UPS, XPS, and Auger spectrometers. Review of Scientific Instruments, 57(9), 2366-.
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  Abstract: A digital data-acquisition system has been built to interface two photoelectron spectrometers to a DEC LSI-11/23 processor.
• Root, D. R., Blevins, C. H., Lichtenberger, D. L., Sattelberger, A. P., & Walton, R. A. (1986). Experimental characterization of an electron-rich (σ²π⁴δ²δ* ²) metal-metal triple bond. Synthesis, reactivity, and photoelectron spectral studies of trimethylphosphine complexes of dirhenium(II). Journal of the American Chemical Society, 108(5), 953-959.
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  Abstract: The reaction of (n-Bu4N)2Re2X8 (X = Cl or Br) with PMe3 gives high yields of the triply bonded complexes Re2X4(PMe3)4. These compounds are oxidized by NOPF6 to give paramagnetic [Re2X4(PMe3)4]PF6, and the chloro derivative reacts with Ph2PCH2PPh2 (dppm) and Ph2PNHPPh2 (dppa) to give Re2Cl4(PMe3)2(dppm) and Re2Cl4(PMe3)2(dppa), respectively. NMR spectroscopy (1H and 31P{1H}) shows that the latter complexes possess fairly symmetrical structures in which the PMe3 ligands are in cis dispositions with respect to the bridging dppm and dppa ligands. The volatility of Re2Cl4(PMe3)4 has permitted the measurement of its gas-phase photoelectron spectrum which accords with this compound possessing a σ2π4δ2δ*2 configuration. The δ* ionization band is slightly narrower than the & ionization band and occurs at about 0.9 eV lower binding energy. The π ionization gives evidence of spin-orbit splitting as expected for the heavy-atom rhenium character. An ionization assigned to removal of an electron from the valence σ orbital is observed at a binding energy 1 eV higher than the π ionization. Comparison of these ionizations with those of the corresponding W2Cl4(PMe3)4 (σ2π4δ2) complex is especially informative. In particular, these observations support a strong interaction between the valence σ density on one metal atom and the core density on the neighboring metal atom in these complexes. © 1986 American Chemical Society.
• Ashby, M. T., & Lichtenberger, D. L. (1985). Cyclic polythioether complexes: Preparation and crystal structure of tricarbonyl(1,4,7-trithiacyclononane)molybdenum(0). Inorganic Chemistry, 24(4), 636-638.
• Kober, E. M., & Lichtenberger, D. L. (1985). The valence σ ionization in systems with multiple metal-metal bonds. Journal of the American Chemical Society, 107(24), 7199-7201.
• Lichtenberger, D. L., & Hubbard, J. L. (1985). Comparison of thionitrosyl and nitrosyl bonding in dicarbonyl(η⁵-cyclopentadienyl)chromium complexes by gas-phase ultraviolet and x-ray photoelectron spectroscopy. Inorganic Chemistry, 24(23), 3835-3841.
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  Abstract: The gas-phase He I/He II UPS and Mg Kα XPS data for the compounds (η5-C5H5)Cr(CO)2NO and (η5-C5H5)Cr(CO)2NS are reported. The first ionization potentials for the two complexes are nearly the same (nitrosyl 7.56 eV; thionitrosyl 7.47 eV), contrary to indications from mass spectrometry appearance potentials. A short C-O stretching progression (ca. 2000 cm-1) in the first ionization band of the nitrosyl compound shows that it is predominantly associated with metal d character and is symmetrically π delocalized into the carbonyls. This orbital is primarily of δ symmetry with respect to the NO or NS ligand. The next two ionizations for both complexes, which complete the formal d6 configuration at the metal, are sensitive to the π symmetry interaction with the NO or NS group. These ionizations are approximately degenerate for the NO complex and are stabilized by nearly 1 eV relative to the first band as a consequence of the stronger Cr-NO π-back-bonding relative to Cr-CO back-bonding. These ionizations are slightly destabilized and are sharper for the NS complex relative to the NO complex, showing stronger metal π interaction with both the NS π bond and π* orbitals. The dramatic loss of intensity of these ionizations in the He II spectrum of the thionitrosyl complex experimentally demonstrates significant delocalization with both the NS π-donor and π-acceptor levels to give large NS (sulfur) character for these ionizations. The nitrogen 1s binding energy for the NS complex is almost 2 eV lower than that for the NO complex, suggesting a much more negative charge on the NS nitrogen. In contrast, the chromium, carbon, and oxygen core binding energies do not differ significantly between the two complexes. A formal potential model analysis of the (Cr-N-O) and (Cr-N-S) XPS shifts indicates a slightly more positive metal in the NS complex. These results are in agreement with Fenske-Hall approximate calculations and with other physical properties of the two compounds. © 1985 American Chemical Society.
• Lichtenberger, D. L., Kellogg, G. E., & Landis, G. H. (1985). Principles of electronic structure in transition metal complexes. Additive ligand electronic effects and core-valence ionization correlations for Mo(CO)_6-n(PMe₃)_n where n=0, 1, 2, 3. The Journal of Chemical Physics, 83(6), 2759-2768.
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  Abstract: Gas phase core photoelectron spectroscopic (XPS) results are reported for a series of trimethylphosphine substituted molybdenum carbonyls: Mo(CO) 6, Mo(CO)5(PMe3), cis-Mo(CO) 4(PMe3)2, trans-Mo(CO)4(PMe 3)2, and fac-Mo(CO)3(PMe3) 3. Core ligand additivity, defined as a constant shift in core ionizations with each successive step of ligand substitution, is indicated by these data. The shift per phosphine substitution is -0.65±0.10 eV for the molybdenum 3d5/2 ionization, -0.75±0.11 eV for the carbon (carbonyl) 1s ionization, and -0.78±0.09 eV for the oxygen 1s ionization. Comparison of core and valence data sets for these complexes illustrates a second principle, core-valence ionization correlation. The ratio of the Coulombic valence metal d level shifts to the core metal shifts is 0.74±0.06. This trend, in a system with extensively delocalized metal orbitals, shows that core and valence photoelectron spectroscopies are intimately related and that key additional understanding of electron distributions and bonding can be obtained from correlating the information of these techniques. Simple models for both the ligand additivity and core-valence ionization correlation principles are presented to demonstrate the fundamental features and possible limitations of these principles. © 1985 American Institute of Physics.
• Bursten, B. E., Darensbourg, D. J., Kellogg, G. E., & Lichtenberger, D. L. (1984). Ligand additivity in the valence photoelectron spectroscopy of phosphine-substituted molybdenum carbonyls. Inorganic Chemistry, 23(25), 4361-4365.
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  Abstract: The He I valence photoelectron spectra in the 6-9-eV ionization region are reported for the following complexes: Mo-(CO)5PMe3, cis-Mo(CO)4(PMe3)2, trans-Mo(CO)4(PMe3)2, trans-Mo(CO)3(PMe3)3, Mo(CO)5PEt3, trans-Mo(CO)4(PEt3)2, Mo(CO)5P-n-Bu3, and trans-Mo(CO)4(P-n-Bu3)2. The observation of vibrational structure due to excitation of a CO stretching mode in several of the spectra aids band assignment. These data are used to test the validity of the model of ligand additivity for determining the energetics of the dπ electrons in octahedral d6 complexes. It is shown that the model successfully correlates the positions, intensities, and number of the ionization bands for the above compounds. A comparison of the results for the complexes of different phosphine ligands indicates that PMe3, PEt3, and P-n-Bu3 are comparable π-acceptors, but that the σ-donor strength increases as PMe3 < PEt3 < P-n-Bu3. © 1984 American Chemical Society.
• Lichtenberger, D. L., & H., C. (1984). Contribution of a δ-orbital electron to a quadruple metal-metal bond. A direct experimental measure from vibrational fine structure in the δ ionization of Mo₂(O₂CCH₃)₄. Journal of the American Chemical Society, 106(6), 1636-1641.
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  Abstract: The first observation of metal-metal vibrational fine structure in a photoelectron band is reported. Attention is focused on the predominantly metal "δ" ionization band (arising from production of the 2B2g positive ion state) of Mo2(O2CCH3)4. From our high-resolution and high signal-to-noise He I ionization data collection techniques, we have observed within this band approximately 20 members of the vibrational progression predominantly corresponding to the totally symmetric (a1g) metal-metal stretching mode in the 2B2g positive ion state. The metal-metal stretching frequency is found to be lower in the 2B2g state than in the ground state, indicating an appreciable reduction of metal-metal bond strength upon the loss of the δ-bonding electron. This stretching frequency is also lower than that observed in the 1A2u (δδ*) electronic excited state of the neutral molecule as obtained from absorption techniques. A normal Franck-Condon analysis of the vibrational progression in the 2B2g positive ion state reveals that the equilibrium metal-metal bond distance is substantially longer than in the ground state of the molecule and, in addition, is longer than in the 1A2u electronic excited state. The comparatively low metal-metal vibrational frequency and long bond length indicate that, in addition to the change in bond order upon ionization, the change in the metal oxidation state also serves an important role in determining the strength of the resultant metal-metal interaction. © 1984 American Chemical Society.
• Lichtenberger, D. L., & Hubbard, J. L. (1984). Thionitrosyl and bridging sulfide complexes of dicarbonyl[hydridotris(3,5-dimethylpyrazolyl)borato]metal (metal = molybdenum and tungsten). Inorganic Chemistry, 23(17), 2718-2720.
• Lichtenberger, D. L., Calabro, D. C., & Kellogg, G. E. (1984). Electronic structure and bonding characteristics of cyclopentadienyl d⁸ metal-ligand complexes. Core and valence ionization study of CpM(CO)₂ where M = Co and Rh and Cp = η⁵-C₅H₅ and η⁵-C₅(CH₃)₅. Organometallics, 3(11), 1623-1630.
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  Abstract: A combined investigation of the core and valence ionizations of d8 cobalt and rhodium complexes of the form CpM(CO)2, where Cp is either η5-cyclopentadienyl or η5-pentamethylcyclopentadienyl, is used to reveal the important electronic interactions in these complexes. It is found that, unlike the usual similarity of ionization features of analogous first- and second-row d6 metal complexes, there are major differences between the valence ionizations of these cobalt and rhodium species. The primary orbital characters associated with each ionization feature are determined by comparing the relative He I/He II intensities, the relative core and valence ionization shifts, and the consistent shifts with ring methylation. Evidence for strong filled-metal with filled-ring orbital interaction is found in the first ionization band and in a band in the region of the cyclopentadienyl e1″ ionizations. Vibrational fine structure is also observed in the first ionization band, revealing substantial back-bonding into the carbonyls. This band shifts only slightly from cobalt to rhodium. The other predominantly metal ionizations increase substantially in ionization energy from cobalt to rhodium, and the splitting of the ionizations originating from the cyclopentadienyl e1″ pair of orbitals more than doubles. In contrast to these large changes in the valence ionizations from cobalt to rhodium, there are essentially no changes in the core ionizations, the ground-state geometrical structures, the carbonyl stretching frequencies, and other physical properties. The combined data strongly indicate that the changes in valence ionizations are largely dependent upon excited-state effects in the positive ions. These results have implications for the comparative stability and chemical behavior of cobalt and rhodium complexes. © 1984 American Chemical Society.
• Lichtenberger, D. L., H., C., & Ortega, R. B. (1984). Distortions in coordinated cyclopentadienyl rings: Crystal, molecular, and electronic structural analysis of (η⁵-pentamethylcyclopentadienyl)dicarbonylrhodium. Organometallics, 3(11), 1614-1622.
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  Abstract: The crystal and molecular structure of Rh(η5-C6(CH3)5)(CO)2 has been determined from a single-crystal X-ray diffraction study. The entire molecule approximates bilateral symmetry with the molecular mirror plane normal to the cyclopentadienyl ring and bisecting the Rh(CO)2 fragment. The cyclopentadienyl C(ring)-C(ring) bond lengths consist of one short bond of 1.384 (8) Å adjacent to two long bonds of 1.445 (8) and 1.447 (7) Å, with the remaining two bonds having intermediate lengths of 1.412 (8) and 1.410 (7) Å. These bond distances are within one standard deviation of the distances found in the analogous cobalt complex. In addition, the cyclopentadienyl ring exhibits a slight symmetric distortion from planarity that significantly reflects the bonding between the ring and the metal. Molecular orbital calculations on an idealized geometry of Rh(η5-C5H5)(CO)2, in which the cyclopentadienyl ring is given D5h, symmetry, illustrate the electronic origin of the ring distortion. Calculated three-dimensional electron density maps show that the bonding of the ring to the metal is slightly dominated by a "dialkene" type interaction in which a single e1- orbital of the ring donates into an empty inplane metal d orbital of the d8 Rh(CO)2+ portion of the molecule. The distortions of the ring are consistent with the character of this orbital. The title compound crystallizes in the monoclinic space group P21/n with a = 7.765 (3) Å, b = 10.741 (3) Å, c = 15.267 (7) Å, β = 98.75 (3)°, and a calculated density of 1.551 g cm-3 for Z = 4. Full-matrix least-squares refinement with varying positional and anisotropic thermal parameters for the non-hydrogen atoms and idealized positional and isotropic thermal parameters for the hydrogen atoms converged at R1 = 0.0339, R2 = 0.0418, and GOF = 1.6818 (n = 136) for the 1610 independent reflections (I ≥ 3σ(I)) collected with Mo Ka radiation over the 2θ range of 4° ≤ 2θ ≤ 50°. © 1984 American Chemical Society.
• Minelli, M., Hubbard, J. L., Lichtenberger, D. L., & Enemark, J. H. (1984). Multinuclear NMR studies of electron distributions: ¹⁴N, ¹³C, and ⁹⁵Mo spectra of nitrosyl and thionitrosyl complexes of chromium, molybdenum, and tungsten. Inorganic Chemistry, 23(17), 2721-2722.
• Piancastelli, M. N., Keller, P. R., Taylor, J. W., Grimm, F. A., Carlson, T. A., Krause, M. O., & Lichtenberger, D. (1984). Trend of shape resonance-induced features in the angular distribution parameter as a function of photon energy for carbon, silicon and germanium tetrac. Journal of Electron Spectroscopy and Related Phenomena, 34(3), 205-214.
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  Abstract: The angular distribution parameter as a function of photon energy, β(hν), has been measured in the 13-27 e V range using synchrotron radiation for the five valence orbitais in silicon and germanium tetrachlorides. The results have been compared with those previously reported for carbon tetrachloride. In particular, the trend of shape resonanceinduced features has been followed along the valence isoelectronic series CCl4-SiCl4-GeCl4 for the 1e and 2t2 orbitals. As the atomic number of the central atom increases, the shape resonances move toward the threshold ionization energy for a particular molecular orbital. © 1984.
• Calabro, D. C., & Lichtenberger, D. L. (1982). Sunlight Photochemistry: The preparation of dicarbonyl(η⁵-methylcyclopentadienyl) triphenylphosphinemanganese. Journal of Chemical Education, 59(6), 686-687.
• Cotton, F. A., Hubbard, J. L., Lichtenberger, D. L., & Shim, I. (1982). Comparative studies of Mo-Mo and W-W quadruple bonds by SCF-Xα-SW calculations and photoelectron spectroscopy. Journal of the American Chemical Society, 104(3), 679-686.
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  Abstract: The homologous compounds M2Cl4(PR3)4, with M = Mo and W, have been used in a combined theoretical and experimental study to compare the electronic structures of quadruple bonds between molybdenum atoms and between tungsten atoms. The theoretical work was carried out by the SCF-Xα-SW method on the model systems with R = H, but using in all other respects the experimentally measured bond lengths and angles for the compounds with R = CH3. Relativistic corrections were made for both the molybdenum and tungsten compounds, but were found to be significant only for the tungsten compound. The PMe3 compounds were used for measurements, made with both He I and He II excitation, of the photoelectron spectra in the gas phase. For both compounds the highest filled orbital is the M-M δ-bonding orbital and the measured ionization energies are 6.44 and 5.81 eV for the Mo and W compounds, respectively. For both compounds the next observed ionizations, at 7.70 eV (Mo) and 7.05, 7.45 eV (W), can be assigned on experimental criteria to the M-M π-bonding orbitals. The spin-orbit splitting of the W-W π peak shows features attributable to mixing of σ, π, and δ components by the spin-orbit coupling operator. These peaks are followed by ionizations assignable to M-P bonding electrons at 8.41 eV (Mo) and 8.36 eV (W). The calculations predict this order correctly for the W compound but reverse the Mo-Mo π and Mo-P ionization energies. The W-W bonding appears to be weaker than the Mo-Mo bonding, and in general the results of this study are consistent with the greater reactivity (i.e., lower chemical stability) of the W-W quadruple bond as compared to the Mo-Mo quadruple bond. © 1982 American Chemical Society.
• Hubbard, J. L., & Lichtenberger, D. L. (1982). Vibrational fine structure in the valence ionizations of transition-metal hexacarbonyls: New experimental indication of metal-to-carbonyl π bonding. Journal of the American Chemical Society, 104(8), 2132-2138.
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  Abstract: The first observations of metal-carbon vibrational structure in photoionization bands are reported. Attention is focused on the predominantly metal d ionizations of M(CO)6 (M = Cr, Mo, and W), and the methods for obtaining high resolution and very high signal-to-noise He I ionization data are detailed. The 2T2g ionization band of Cr(CO)6 and the spin-orbit split 2E″ and 2U′ bands of W(CO)6 show distinct vibrational progressions which correspond to the totally symmetric (a1g) metal-carbon stretching mode in the positive ion states. The metal-carbon stretching frequencies are found to be significantly less in the positive ion states than in the ground states, indicating a reduction of metal-carbon bond order upon the loss of a t2g electron. Evaluation of the vibrational progressions shows that the metal-carbon bond length increases on the order of 0.10 Å upon t2g ionization in the case of W(CO)6 and about 0.14 Å in the case of Cr(CO)6. In addition, the beginning of a short progression in the a1g carbon-oxygen stretching mode is observed in the Mo(CO)6 spectrum and is clearly seen in the W(CO)6 spectrum. All of these observations show that removal of an electron from the predominantly metal t2g orbitals, which are strictly π symmetry with respect to the carbonyls, substantially weakens the metal-to-carbonyl bond. © 1982 American Chemical Society.
• Calabro, D. C., & Lichtenberger, D. L. (1981). Valence ionizations of olefins coordinated to metals. Olefin dicarbonyl(η⁵-(methyl and pentamethyl)cyclopentadienyl)manganese complexes. Journal of the American Chemical Society, 103(23), 6846-6852.
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  Abstract: The He I and He II valence photoelectron spectra of (η5-C5H5-n(CH3) n)Mn(CO)2L (n = 1 and 5; L = C2H4 and C3H6) are presented. The synthesis of the pentamethylcyclopentadienyl metal ethylene complex is reported for the first time. This complex is particularly helpful in revealing the ionization characteristics of the metal-olefin complexes. In each case the ionization that is associated primarily with the olefin π bond (2e- donor to the metal) is shifted to lower binding energy in the complex compared to the binding energy of the free ligand. Molecular orbital calculations are reported that agree very well with the shifts in ionization energies. The relative magnitudes of individual metal-olefin orbital interactions are evaluated. It is found that the ionizations and stability of the complexes are sensitive to the geometry changes that accompany coordination of the olefin. These distortions are associated with a lowered carbon-carbon bond strength and an increased metal-olefin bond strength through increased π-donor/π*-acceptor interactions. © 1981 American Chemical Society.
• Calabro, D. C., Hubbard, J. L., H., C., Campbell, A. C., & Lichtenberger, D. L. (1981). The effects of methyl group substitution on metal-coordinated cyclopentadienyl rings. The core and valence ionizations of methylated tricarbonyl(η⁵-cyclopentadienyl)metal complexes. Journal of the American Chemical Society, 103(23), 6839-6846.
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  Abstract: Gas-phase He I, He II, and Mg Kα photoelectron spectra are reported for molecules of the type (η5-C5H5-n-(CH3)n)M(CO) 3 where n = 0, 1, 5 and M = Mn, Re. The influence of methyl groups on the cyclopentadienyl ring is monitored by shifts in both core and valence ionization energies. This enables effective separation of electron density transfer (inductive) and ring-methyl orbital overlap (hyperconjugative) effects. While the shift in the ring e1″ ionization is found to be primarily a hyperconjugative effect, the shift in the metal valence ionizations is caused essentially entirely by a shift of electron density toward the metal atom. A greater proportion of this increased density is transferred to the carbonyls in the rhenium complexes than in the manganese complexes, indicating the greater back-bonding ability of the third-row atom. Further evidence of extensive Re-CO back-bonding is provided by the presence of vibrational fine structure on one of the predominantly metal ionizations of the rhenium complexes. This structure is the vibrational progression of the symmetric metal-carbon(CO) stretching mode. The long vibrational progression observed in this band and the frequency of the M-C stretch in the positive ion are direct evidence of considerable π back-bonding from the metal to the carbonyls. The observed vibrational structure in the spin-orbit split rhenium d ionizations also leads to a definitive interpretation of the pattern of metal ionizations in such complexes. The origin of the characteristic splitting of the predominantly ring e1″ ionization is also considered in detail. The data suggest that the carbon-carbon bond distances in the ring are distorted an average of 0.01 to 0.02 Å from fivefold symmetry when coordinated to a d6 ML3 species. This is the first indication from gas-phase spectroscopy for such distortions. © 1981 American Chemical Society.
• Calabro, D. C., Lichtenberger, D. L., & Herrmann, W. A. (1981). The valence electronic structure of bridging methylenes: UV photoelectron spectroscopy of μ-methylene-bis(dicarbonyl(η⁵-cyclopentadienyl)manganese). Journal of the American Chemical Society, 103(23), 6852-6855.
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  Abstract: The He I photoelectron spectrum of μ-CH2-[(η5-C5H4CH 3)Mn(CO)2]2 in the ionization energy range below 11 eV is reported and compared with the ionizations of (η5-C5H4CH3)Mn(CO)3 and (η5-C5H4CH3)Mn(CO) 2(C2H4). Excellent agreement is found between the observed ionizations and the predictions of parameter-free molecular orbital calculations. The valence orbitals of the μ-CH2 group appear to have near ideal matching with the frontier orbitals of the (η5-C5H4CH3)Mn(CO)2 fragments to produce the bonding and stability of this cyclopropane analogue. An effective charge transfer from the metals to the methylene occurs in this interaction which results in a high negative charge on the methylene carbon and formation of a net metal-metal bond. The bonding of the bridging methylene in this complex is also compared with the bonding of a terminal methylene with the analogous (η5-C5H5)Mn(CO)2 species. It is concluded that the formation of the metal-metal bond is an important factor in the greater stability of the bridging system. © 1981 American Chemical Society.
• Chandler, T., Lichtenberger, D. L., & Enemark, J. H. (1981). Bonding in the syn and anti isomers of di-μ-sulfido-bis(sulfido(1,2-dimercaptoethanato)molybdate(V)) anions. Inorganic Chemistry, 20(1), 75-77.
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  Abstract: The electronic structure and bonding interactions in the syn, anti, and closed isomers of [Mo2S4(S2C2H4) 2]2- are examined and compared through extended Hückel and Fenske-Hall molecular orbital calculations. A molybdenum-molybdenum bonding interaction is found to account for the diamagnetism of the complexes and to be important in determining the relative stability of the isomers. The predicted stabilities are syn > closed > anti. © 1981 American Chemical Society.
• Hubbard, J. L., & Lichtenberger, D. L. (1981). The Jahn-Teller effect in the photoelectron spectrum of iron pentacarbonyl. The Journal of Chemical Physics, 75(6), 2560-2568.
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  Abstract: High quality photoelectron spectra of gaseous Fe(CO)5 excited by HeI, HeII, and ArI photons have been obtained. Major attention is focused on the primarily metal 3d ionizations, which occur in the binding energy region from 7 to 11 eV. Ionization to the 2E′ positive ion state (centered at 8.6 eV) clearly shows the effects of Jahn-Teller distortions in the positive ion. This ionization results in two ionization bands of approximately equal intensity and shape separated by 0.38 eV at room temperature. These bands broaden and the splitting increases to 0.47 eV at 473 °K. Ionization to the 2E″ positive ion state, centered at 9.9 eV, is much less influenced by the Jahn-Tellar effect. There is no discernable splitting of this band at room temperature. These observations are discussed in terms of the electronic structure and bonding of Fe(CO)5. Simple model calculations of the energies of the doubly degenerate electronic states in relation to the appropriate doubly degenerate normal vibrational modes are used to investigate the splitting. The magnitudes of the 2E′ and 2E″ splittings are reproduced very well, and the temperature dependence is reproduced within experimental error. It is found that the vibrational motion primarily responsible for the splitting is a low frequency OC-Fe-CO bending in the equatorial plane. This bending is found in two normal modes, one of which is closely related to the Berry pseudorotation process. The splitting of the 2E′ ionization is clear experimental evidence in the gas phase that the preferred geometry of d7 M(CO)5 is not D3h in this case. © 1981 American Institute of Physics.
• Calabro, D. C., & Lichtenberger, D. L. (1980). Comments on Koopmans' theorem and electron relaxation with valence ionization. Inorganic Chemistry, 19(6), 1732-1734.
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  Abstract: The electron relaxation energy, ER, is a well-recognized limitation of Koopmans' approximation for ionization energies. Several characteristic aspects of ER for first ionizations of atoms are discussed. It is shown that (1) Slater's screening model gives good account of the basic trends in ER with valence ionizations of main-group atoms but fails for d electron ionizations of transition metals, (2) the relatively large ER for ionization of 3d electrons is association with the 3d shielding of 3s and 3p electrons (these relaxation characteristics help to clarify the filling of the 4s and 3d orbitals in the periodic table), and (3) there is more than a proportional decrease in ER when the valence density removed from an atom is less than a complete electron (this indicates that the intraatomic portion of ER will be less for ionization from a delocalized orbital than from a localized orbital). © 1980 American Chemical Society.
• Calabro, D. C., & Lichtenberger, D. L. (1980). Erratum: Comments on Koopmans' theorem and electron relaxation with valence ionisation (Inorganic Chemistry (1980) 19, (1732-1733)). Inorganic Chemistry, 19(11), 3552-.
• Hubbard, J. L., & Lichtenberger, D. L. (1980). Counterintuitive consequences of filled ligand π interactions with metal d orbitals [1]. Inorganic Chemistry, 19(12), 3865-3866.
• Hubbard, J. L., & Lichtenberger, D. L. (1980). Photoelectron spectra of nitrosyldicarbonyl(η⁵-cyclopentadienyl)chromium and thionitrosyldicarbonyl(η⁵-cyclopentadienyl)chromium. Comparison of the electronic structures of metal-NO and metal-NS complexes. Inorganic Chemistry, 19(5), 1388-1390.
• E., B., Hoffmann, R., & Lichtenberger, D. L. (1979). CpM(CO)₂(ligand) complexes. Journal of the American Chemical Society, 101(3), 585-591.
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  Abstract: The electronic structure of cyclopentadienyl metal dicarbonyl complexes of alkyls, carbenes, sulfur dioxide, acetylenes, and ethylenes is analyzed, with an emphasis on conformational preferences and rotational barriers. © 1979 American Chemical Society.
• Lichtenberger, D., & Hubbard, J. (1979). Are ionization energies right-handed or left-handed?. Journal of Electron Spectroscopy and Related Phenomena, 16(1), 123-.
• McPherson, A. M., Fieselmann, B. F., Lichtenberger, D. L., McPherson, G. L., & Stucky, G. D. (1979). Electronic properties of bis(η⁵-cyclopentadienyl)titanium 2,2′-bipyridyl. A singlet molecule with a low-lying triplet excited state. Journal of the American Chemical Society, 101(13), 3425-3430.
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  Abstract: Magnetic susceptibility and EPR studies show that (η5-C5H5)2Ti(bpy) has a triplet excited state which is thermally accessible from the ground-state singlet. The temperature dependence of the magnetic susceptibility and the EPR spectra was measured and confirms that the singlet and triplet states are separated by approximately 600 cm-1 in toluene solution and 750 cm-1 in the solid state. In both solution and solid state (η5-C5H5)2T(bpy) is monomeric so that the presence of extended Ti-Ti interactions is unlikely. The low-lying triplet most likely corresponds to a state in which one unpaired electron formally occupies a molecular orbital which is localized on (η5-C5H5)2Ti while a second unpaired electron resides in the lowest energy π* orbital of the bipyridyl group. This conclusion is supported by Fenske-Hall molecular orbital calculations. The calculations and single crystal X-ray data also suggest that the preferred molecular geometries of the singlet and triplet states are not the same. The cationic complex formed by the one-electron oxidation of (η5-C5H5)2Ti(bpy) has been isolated as a hexafluorophosphate salt. The magnetic susceptibility and EPR spectrum of this material are characteristic of a d1 complex. © 1979 American Chemical Society.
• Lichtenberger, D. L., & Brown, T. L. (1978). Cis labilization of ligand dissociation. 5. A molecular orbital investigation. Journal of the American Chemical Society, 100(2), 366-373.
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  Abstract: The process of ligand dissociation from a transition metal carbonyl center has been examined using a nonempirical molecular orbital approach. The energy requirements for CO dissociation from Mn(CO)6+, and from Mn(CO)5X, where X = Br or H, have been investigated. Consideration of the interaction between individual CO groups and the remainder of the molecule in the ground state structures of these species does not provide a rationale for the relative rates of CO dissociations. Instead, the relative energies for CO dissociation appear to depend upon the relative energies of stabilization resulting from geometrical relaxation of the metal carbonyl fragment which remains following CO loss. The cis CO groups of Mn(CO)5Br are found to be more labile than the trans CO group or than the CO groups of Mn(CO)6+, because in formation of the transition state more stabilization energy derives from relaxation of the Mn(CO)4Br fragment. Ligands which have at least one potential π-donor orbital labilize the cis carbonyl groups to the greatest extent, as a result of an improved bonding interaction in the five-coordinate fragment. The calculations suggest that the σ-bonding ability of a ligand such as H is less influential than π-bonding characteristics. © 1978 American Chemical Society.
• Lichtenberger, D. L., & Brown, T. L. (1978). Nonbridged structures of dicobalt octacarbonyl [2]. Inorganic Chemistry, 17(5), 1381-1382.
• Lichtenberger, D. L., & Brown, T. L. (1977). A variable temperature carbon-13 nuclear magnetic resonance investigation of intramolecular rearrangement in (CO)₄CoEX₃ complexes. Journal of the American Chemical Society, 99(25), 8187-8194.
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  Abstract: The temperature-dependent 13C NMR spectra for 12 compounds of the form (CO)4CoEX3 (where E = C, Si, Ge, Sn, or Pb and X = F, Cl, CH3, C4H9, CH2C6H5, or C6H5) in CFCl2H solution have been recorded. At higher temperatures a single 13C carbonyl resonance is observed for all molecules, indicating a time averaging of the axial and radial carbonyl signals due to an intramolecular exchange process. At lower temperatures the exchange process is observed to slow; coalescence temperatures range from less than -160°C when EX3 = SnCl3 to -10°C when EX3 = CF3. Complete line shape fitting of the spectra permits determination of the activation parameters for the exchange for most of the compounds studied. The results indicate that the free energy barriers to intramolecular rearrangement, which range from about 4.5 to 11.8 kcal/mol, are determined mainly by the steric requirements of the EX3 group. The barrier increases with increasing steric requirement of the EX3 group, as measured by the van der Waals contact angle with adjacent CO groups. Enthalpies of activation are in the range 6-7 kcal/mol for trihalogeno groups, 8.3-9.3 kcal/mol for E(C6H5)3 groups, and 7-8 kcal/mol for trialkyltin groups. The experimental observations are discussed in the light of possible mechanisms for the intramolecular exchange.
• Lichtenberger, D. L., & Fenske, R. F. (1976). Electronic structure of transition metal thiocarbonyl complexes. Inorganic Chemistry, 15(9), 2015-2022.
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  Abstract: The He I photoelectron spectra of Cr(CO)5CS, W(CO)5CS, and η5-C5H5Mn(CO)2CS have been obtained. It is observed that the ionizations which are associated predominantly with the highest occupied σ and π levels of the thiocarbonyl ligand are clearly separated from the other ionizations of these complexes and display quite different ionization band envelopes. The remaining ionization bands, of these complexes are comparable to the bands observed in the ionization spectra of the corresponding carbonyl complexes. Molecular orbital calculations were performed on the chromium and manganese species. Besides predicting the correct order for the ionizations, the calculations are highly successful in giving a quantitative account of observed trends in ionization potentials between the carbonyl complexes and the thiocarbonyl complexes. As suspected, the thiocarbonyl ligand is found to be a better π-electron acceptor than the carbonyl ligand. However, consideration of this factor alone does not account for the observed trends in ionization potentials between carbonyl and thiocarbonyl complexes. The occupied thiocarbonyl σ and π levels also interact more strongly with the metal, and the electron density has increased at the carbon of the thiocarbonyl group. These factors have important implications for the properties of thiocarbonyl complexes.
• Lichtenberger, D. L., & Fenske, R. F. (1976). The helium(I) photoelectron spectra and electronic structure of (η⁵-cyclopentadienyl) d⁶ metal carbonyls. Journal of the American Chemical Society, 98(1), 50-63.
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  Abstract: The He(I) photoelectron spectra of CpMn(CO)3, CpRe(CO)3, and CpFe(CO)2X, where Cp = η5-C5H5 and X = Cl, Br, I, and CH3, are presented. The low energy ionization bands (binding energies from 7 to 13 eV) are found to be sensitive to metal and ligand substitution. The ionizations which are associated primarily with the cyclopentadienyl ring π e1″ levels display a characteristic band envelope. Interestingly, the degeneracy of the ionizations associated predominantly with the metal d levels in octahedral M(CO)6 complexes and C4v Mn(CO)5X complexes is not substantially removed in these lower symmetry CpM(CO)3 and CpFe(CO)2X analogues. In the case of CpRe(CO)3, a distinct spin-orbit splitting of the predominantly metal ionizations is observed. Comparison of the spectra of the CpFe(CO)2X complexes with the spectra of the corresponding Mn(CO)5X complexes provides additional information regarding the assignment of the valence ionization bands of the molecules in both of these series. An ab initio calculation on the cyclopentadienide ion and approximate calculations on the transition metal complexes are used to aid in the interpretation of these ionizations. In addition, the details of a method for interpreting the ionizations of a molecule containing an atom with appreciable spin-orbit interaction are presented, and applied to the spectrum of CpRe(CO)3. The combined knowledge provided by this work and earlier investigations leads to a consistent description of the electronic structure and bonding of d6 metal η5-cyclopentadienyl carbonyls and indicates that serious errors may result if ionization potentials are interpreted solely on the basis of Koopmans' theorem. The lowered carbonyl force constants in these π-bonded ring complexes (when compared to the corresponding M(CO)6 and Mn(CO)5X complexes) are not found to be a result of electron donation from the ring, as previously believed. Instead, the carbonyl ligands and the ring interact with metal orbitals of primarily different symmetry, and the lower carbonyl force constants are a direct result of a decrease in competition for the back-bonding electrons from the metal orbitals of appropriate symmetry.
• Lichtenberger, D. L., & Fenske, R. F. (1976). The transferability of molecular fragment canonical orbitals. The Journal of Chemical Physics, 64(11), 4247-4264.
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  Abstract: The ability of certain canonical orbitals of isolated molecular fragments to transfer largely unchanged to the molecular environment is examined. The separation of fragment canonical orbitals from the total molecular electronic problem is compared with the more familiar separations of atomic core orbitals and fragment localized orbitals. The specific example of the carbonyl functional group in formaldehyde is examined in detail. These studies lead to a new concept of valence electron only calculations in which the molecular valence electrons are assumed to move in an effective field provided by frozen molecular fragment canonical cores. In addition, for the case of assumed fragment canonical orbital transfer, perturbation theory analysis is found to be an efficient method of assessing the quality of the approximate wavefunction, thus eliminating much of the uncertainty as new systems are studied. The methods developed in the course of these studies offer certain practical advantages for the construction of approximate wavefunctions for large molecules. The details of application of these concepts to existing molecular orbtial methods are also presented. Copyright © 1976 American Institute of Physics.
• Lichtenberger, D. L., Kidd, D. R., Loeffler, P. A., & Brown, T. L. (1976). ⁵⁹Co nuclear quadrupole resonance spectra and low temperature ¹³C magnetic resonance spectra of X₃SnCo(CO)₄ compounds [16]. Journal of the American Chemical Society, 98(2), 629-630.
• Lichtenberger, D. L., Sellmann, D., & Fenske, R. F. (1976). The He(I) photoelectron spectra and valence electronic structures of η⁵-C₅H₅Mn(CO)₂N₂ and η⁵-C₅H₅Mn(CO)₂NH₃. Journal of Organometallic Chemistry, 117(3), 253-264.
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  Abstract: The He(I) photoelectron spectra of η5-C5H5Mn(CO)2N2 and η5-C5H5Mn(CO)2NH3 have been obtained. The general features of these spectra resemble those of the parent carbonyl complex, η5-C5H5Mn(CO)3. The major differences appear in the ionizations associated predominantly with the metal d levels, where shifts in ionization energies and loss of degeneracy reflect the differences in bonding of the nitrogen ligands and a carbonyl ligand with the metal center. Non-empirical molecular orbital calculations were used as an aid for the interpretation of these binding energy shifts. In the case of N2 bound to the metal, the shifts in ionization potentials are predicted with extreme accuracy by the shifts in eigenvalues of the calculations. Thus the electronic structure of transition metal dinitrogen complexes, as compared to carbonyl complexes, is accurately described. The quantitative prediction of binding energy shifts for the amine complex are less satisfactory, although the qualitative behavior is reproduced quite well. © 1976.
• Petersen, J. L., Lichtenberger, D. L., Fenske, R. F., & Dahl, L. F. (1975). Nonparameterized molecular orbital calculations and photoelectron spectroscopy of open- and closed-shell M(IV) M(η⁵-C₅H₅)₂L₂ complexes. Journal of the American Chemical Society, 97(22), 6433-6441.
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  Abstract: Nonparameterized (Fenske-Hall)-type molecular orbital calculations have been performed on several d0, d1, and d2 M(IV) M(η5-C5H5)2L2 molecules, and the results have been found to be completely compatible with the interpretations obtained from the electron paramagnetic resonance and photoelectron spectral measurements as well as from the crystallographically acquired bond-length and bond-angle data. The MO calculations reveal that the LUMO for the d0 Ti(IV) complexes and the HOMO for the d1 and nonhydridic d2 M(IV) complexes possess analogous orbital characters principally associated with the metal dz2 and dx2-y2 AO's with considerable contributions from the 3p AO's of the sulfur or chlorine L ligands. The relative metal orbital compositions of the HOMO's in the open-shell d1 V(IV) complexes are in remarkable agreement with those obtained from our previously reported dilute single-crystal EPR studies on V(η5-C5H5)2S5 and V(η5-C5H4CH3)2Cl 2. For V(η5-C5H5)2S5 the per cent character ratio of 3dz2/3dx2-y2 = 12.7/1 (EPR) vs. 7.7/1 (MO), while the corresponding ratio of 20.0/1 from the EPR data on V(η5-C5H4CH3)2Cl 2 compares with that of 20.5/1 from the MO computations. Additional information concerning the electronic structure of M(η5-C5H5)2L2 complexes has been obtained from photoelectron spectroscopy. The PE spectra of M(η5-C5H5)2Cl2 and M(η5-C5H4CH3)2Cl 2 (where M = Ti, V) are presented and interpreted with the aid of the approximate MO calculations.
• Whitesides, T. H., Lichtenberger, D. L., & Budnik, R. A. (1975). Bonding in ring whizzers. I. Photoelectron spectra and molecular orbital calculations for (η⁵-C₆H₇)Mn(CO)₃, (η⁵-C₇H₉)Mn(CO)₃, and (η⁵-C₇H₇)Mn(CO)3. Inorganic Chemistry, 14(1), 68-73.
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  Abstract: A series of calculations, checked by reference to photoelectron spectral (PES) data, is used to interpret the electronic structure and elucidate the principal bonding interactions in a series of cyclic pentadienylmanganese tricarbonyl complexes. A linear correlation is obtained between the calculated eigenvalues and the observed ionization potentials. This correlation suggests that deviations from Koopman's theorem are reasonably constant in these complexes and that the PES data can be adequately assigned by the results of the calculations. Two important conclusions result from this study. (1) The principal bonding interaction between the organic ligand and the metal is due to the interaction of the a′(e1″) level of the pentadienyl moiety with the metal dxz orbital; this interaction is considerably more important than the a″(e1″)-dyz interaction. (2) Calculations indicate a low-lying virtual level in C7H7Mn(CO)3, resulting from the interaction of the π* level with a″(e2″) which may play an important role in the fluxional degenerate rearrangement of this molecule.
• Lichtenberger, D. L., & Fenske, R. F. (1974). Assignment of the photoelectron spectra of Mn(CO)₅CH₃ and Mn(CO)₅CF₃. Inorganic Chemistry, 13(2), 486-488.
• Lichtenberger, D. L., Sarapu, A. C., & Fenske, R. F. (1973). Photoelectron spectra and electronic structure of pentacarbonylmanganese halides. Inorganic Chemistry, 12(3), 702-705.

Proceedings Publications

• Hamilton, M. O., Karayilan, M., Glass, R. S., Pyun, J., & Lichtenberger, D. L. (2020, 2020///). Mechanistic investigations of [2Fe-2S] clusters for the electrocatalytic production of hydrogen.. In Abstracts of Papers, 259th ACS National Meeting & Exposition, Philadelphia, PA, United States, March 22-26, 2020, INOR-1010.
• Lichtenberger, D. L., Brezinski, W. P., Karayilan, M., Clary, K., Hamilton, M. O., Glass, R. S., & Pyun, J. (2020, 2020///). Design principles for fast electron and proton transfer yielding enhanced electrocatalytic production of H2 from pH 7 water.. In Abstracts of Papers, 259th ACS National Meeting & Exposition, Philadelphia, PA, United States, March 22-26, 2020, INOR-0594.
• Clary, K., Karayilan, M., Glass, R. S., Pyun, J., & Lichtenberger, D. L. (2019, 2019///). Synthetic [2Fe2S] biomimietics in metallopolymers as hydrogen evolution electrocatalysts.. In Abstracts of Papers, 258th ACS National Meeting & Exposition, San Diego, CA, United States, August 25-29, 2019, INOR-0400.
• Hamilton, M. O., Humphries, M., & Lichtenberger, D. L. (2019, 2019///). Super-electron-donor dimetal complexes acting as frustrated Lewis pairs: Electrocatalytic production of hydrogen from weak acids.. In Abstracts of Papers, 257th ACS National Meeting & Exposition, Orlando, FL, United States, Mar. 31-Apr. 4, 2019, INOR-0818.
• Hamilton, M. O., Kiselka, J. M., Karayilan, M., Glass, R. S., Pyun, J., & Lichtenberger, D. L. (2019, 2019///). Electrochemical and computational investigation of aromatically bridged [2Fe-2S] clusters for electrocatalytic hydrogen production.. In Abstracts of Papers, 258th ACS National Meeting & Exposition, San Diego, CA, United States, August 25-29, 2019, INOR-0603.
• Hamilton, M. O., Pitts, H. W., Glass, R. S., & Lichtenberger, D. L. (2019, 2019///). Electrocatalytic hydrogen production via aromatically bridged butterfly [2Fe-2S] clusters.. In Abstracts of Papers, 257th ACS National Meeting & Exposition, Orlando, FL, United States, Mar. 31-Apr. 4, 2019, INOR-0704.
• Karayilan, M., Brezinski, W. P., Clary, K., McCleary-Petersen, K. C., Lichtenberger, D. L., Glass, R. S., & Pyun, J. (2019, 2019///). Macromolecular engineering of electrocatalytic metallopolymers via ATRP: Artificial enzymes for water splitting.. In Abstracts of Papers, 257th ACS National Meeting & Exposition, Orlando, FL, United States, Mar. 31-Apr. 4, 2019, POLY-0539.
• Pyun, J., Karayilan, M., Clary, K., Lichtenberger, D. L., & Glass, R. S. (2019, 2019///). Artificial enzymes via ATRP from [2Fe-2S] metallopolymers for H2 production via water splitting.. In Abstracts of Papers, 257th ACS National Meeting & Exposition, Orlando, FL, United States, Mar. 31-Apr. 4, 2019, POLY-0171.
• Brezinski, W., Karayilan, M., Clary, K., Matyjaszewski, K., Evans, D., Lichtenberger, D., Glass, R., & Pyun, J. (2018, MAR 18). [Fe-Fe]-hydrogenase mimetic metallopolymers: Water solubility, enhanced rates, and aerobic stability in hydrogen evolution catalysts via polymer support. In ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 255.
• Clary, K., Petersen, H., Evans, D., Glass, R., & Lichtenberger, D. (2018, MAR 18). Understanding the fast rates of open butterfly [2Fe-2S] cluster-based catalysts for the hydrogen-evolving reaction (HER). In ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 255.
• Humphries, M., Smith, B., Wusterbarth, E., Njardarson, J., & Lichtenberger, D. (2018, MAR 18). Tunable dimetal tetraguanidinate paddlewheel complexes with unprecedented electron donor abilities. In ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 255.
• Karayilan, M., Brezinski, W., Lichtenberger, D., Glass, R., & Pyun, J. (2018, MAR 18). Synthesis of [FeFe]-hydrogenase mimetic metallopolymers via ATRP for robust and O2 resistant H2-evolution catalysts. In ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 255.
• Talbot, M., Evans, D., Glass, R., & Lichtenberger, D. (2018, MAR 18). Electrocatalytic hydrogen production by bridged butterfly [2Fe-2S] catalysts. In ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 255.
• Brezinski, W., Karayilan, M., Lichtenberger, D., Glass, R., & Pyun, J. (2017, 2017///). [Fe-Fe] hydrogenase mimetic metallopolymers from ATRP with long-lived electrocatalytic activity.. In Abstracts of Papers, 254th ACS National Meeting & Exposition, Washington, DC, USA, August 20-24, 2017, POLY-273.
• Brezinski, W., Talbot, M. O., Yamamoto, T., Sill, S., Stratton, L. M., Lichtenberger, D. L., Evans, D. H., & Glass, R. S. (2017, 2017///). Synthesis and characterization of thiophene annulated butterfly [2Fe-2S] complexes.. In Abstracts of Papers, 253rd ACS National Meeting & Exposition, San Francisco, CA, United States, April 2-6, 2017, CATL-163.
• Clary, K. E., In-noi, O., Marx, J. M., Haller, K. J., Evans, D. H., Glass, R. S., & Lichtenberger, D. L. (2017, 2017///). Butterfly [2Fe-2S] cluster-based electrocatalysts with fast rates for hydrogen production.. In Abstracts of Papers, 253rd ACS National Meeting & Exposition, San Francisco, CA, United States, April 2-6, 2017, INOR-879.
• Enemark, J. H., Stein, B. W., Yang, J., Mtei, R., Wiebelhaus, N., Kersi, D., Lichtenberger, D. L., & Kirk, M. L. (2017, 2017///). Metallodithiolenes revealed as unique chemical chameleons.. In Abstracts of Papers, 254th ACS National Meeting & Exposition, Washington, DC, USA, August 20-24, 2017, INOR-945.
• Humphries, M., Wusterbarth, E., Smith, B. R., Njardarson, J. T., & Lichtenberger, D. L. (2017, 2017///). Tunable catalysis with dimetal tetraguanidinate paddlewheel complexes.. In Abstracts of Papers, 253rd ACS National Meeting & Exposition, San Francisco, CA, United States, April 2-6, 2017, INOR-876.
• Lichtenberger, D. L. (2017, April 2-6). Extremely fast electrocatalytic hydrogen production with butterfly [2Fe-2S] clusters inspired by [FeFe]-hydrogenases. In Abstracts of Papers, 253rd ACS National Meeting & Exposition, San Francisco, CA, United States, April 2-6, 2017.
• Lichtenberger, D. L., Brezinski, W., Evans, D. H., Glass, R. S., Karayilan, M., Matyjaszewski, K., & Pyun, J. (2017, October). [Fe-Fe]-hydrogenase mimetic metallopolymers: Water solubility, enhanced rates, and aerobic stability in hydrogen evolution catalysts via polymer support. In Abstracts of Papers, 254th ACS National Meeting & Exposition, New Orleans, LA, United States, March 18-22, 2018.
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  A small molecule [FeFe]-hydrogenase mimic has been synthesized and used to initiate atom transfer radical polymerization (ATRP) of various monomers to generate metallopolymers. These polymers contain active [2Fe-2S] sites which serve as electrocatalysts for the hydrogen evolution reaction (HER) 2H+ à H2. Polymers of the desired molecular weights and low polydispersity were obtained using Cu(I) catalysts and active nitrogen ligands at low temperatures. Growing a polymer chain from both sides of the molecule creates a ‘buried’ active site environment analogous to the protein architecture seen of the [FeFe]-H2ase enzyme. The correct choice of monomer provides water solubility and improves proton transport to facilitate the flow of H+ to the active site in a medium relevant for large scale hydrogen production, leveraging the supramolecular architecture to improve catalysis. IR spectroscopy was used to confirm retention of the [2Fe-2S] moiety and estimate the [Fe2S2(CO)6] concentration in the polymer. Gel permeation chromatography with UV-Vis detection at 400 nm confirms covalent attachment of the [2Fe-2S] system to the polymer. Cyclic voltammetry is used to assess the rate of catalysis defined by a turnover frequency (TOF), and the thermodynamic efficiency of catalysis in terms of overpotential (η). HER electrocatalysts have been prepared using this methodology which display extremely high current densities and low overpotentials in aqueous media at pH 7 as well as a high degree of aerobic stability.
• Lichtenberger, D. L., Clary, K., Evans, D. H., & Petersen, H. (2017, October). Understanding the fast rates of open butterfly [2Fe-2S] cluster-based catalysts for the hydrogen-evolving reaction (HER). In Abstracts of Papers, 254th ACS National Meeting & Exposition, New Orleans, LA, United States, March 18-22, 2018.
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  A viable catalyst toward the production of molecular hydrogen is needed to meet future demand and allow for the possibility of a hydrogen fuel economy. Drawing inspiration from nature in the pursuit of a hydrogen evolving catalyst, worldwide interest has gone into mimicking the activity of hydrogenase enzymes, the most active of which feature a butterfly [2Fe-2S] cluster active site. A class of synthetic [2Fe-2S] cluster-based catalysts that exhibit impressively high rates for the catalytic production of hydrogen on the order of 1000 times faster than the enzyme will be presented. Interestingly, cyclic voltammetry based kinetics experiments show the rate of catalysis for the butterfly [2Fe-2S] complexes are highly dependent on reaction conditions that mitigate competing unproductive mechanistic pathways. Based on electrochemical studies and DFT computations, the steric and electronic factors that lead to the fast rates for these open butterfly [2Fe-2S] cluster-based catalysts will be discussed.
• Lichtenberger, D. L., Humphries, M., Njardarson, J. T., Smith, B. R., & Wusterbarth, E. (2017, October). Tunable dimetal tetraguanidinate paddlewheel complexes with unprecedented electron donor abilitie. In Abstracts of Papers, 254th ACS National Meeting & Exposition, New Orleans, LA, United States, March 18-22, 2018.
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  W2- and Mo2(bicyclic-tetraguanidinate) paddlewheel complexes have unprecedented electron donor abilities that could be applied to many chemical processes, including the catalytic reduction of protons to produce hydrogen and new synthetic strategies in dehalogenation of alkyl and aryl chlorides. Electrocatalytic reduction of protons with W2(TEhpp)4 1, has rates exceeding that of the hydrogenase active site. Additionally, reduction studies with 1 have shown that low temperature single electron transfers on molecules like dichlorobenzene are possible and quantitative. Investigation of Mo2(TEhpp)4 2, an analogue of 1 with a more positive oxidation potential, has shown similar reactivity towards dichloromethane and polychlorinated benzene. Various kinetic studies have been performed elucidating the thermodynamics of the single electron transfer using EPR spectroscopy. Computations indicate that a singlet-triplet electronic energy state crossing of an alkyl chloride associated with the metal complex is operable at low temperature in the dehalogenation reaction.
• Lichtenberger, D. L., In-noi, O., Haller, K. J., Roberts, S. A., & Glass, R. S. (2017, October). Two Conformational Isomers Coxisting in Crystal Lattice of [(µ-SCH3)(µ-SC6H5)Fe2(CO)6]. In 43rd Congress on Science and Technology of Thailand (STT43).
• Lichtenberger, D. L., Karayilan, M., Brezinski, W. P., Glass, R. S., & Pyun, J. (2017, October). Synthesis of [FeFe]-hydrogenase mimetic metallopolymers via ATRP for robust and O2 resistant H2-evolution catalysts. In Abstracts of Papers, 254th ACS National Meeting & Exposition, New Orleans, LA, United States, March 18-22, 2018.
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  Metal-containing polymers can combine the useful properties of polymers with the key functions of metal complexes. These metallopolymers are applicable to a wide range of areas such as photovoltaics, stimuli responsive materials and catalysis. Catalysis, for example, includes designing artificial metalloenzymes which can mimic the biological functionalities by engineering the environment of a metal complex using polymeric materials. FeFe-hydrogenase enzyme found in bacteria is an efficient H2 generation catalyst and there has been extensive research on making FeFe-H2ase mimics to produce H2 as a carbon-free energy carrier. The mimics have shown high catalytic activities in organic media, however, limited lifetime, low oxygen stability and low solubility preclude the applicability of the mimics. We, for the first time, made a metalloinitiator from a FeFe-H2ase mimic to grow polymers via atom transfer radical polymerization (ATRP). The polymers not only provide water solubility and oxygen stability in neutral water but also enhance the activity of the complex by tuning the secondary coordination sphere of the mimic. We will discuss our most recent efforts to synthesize a difunctional metalloinitiator and metallopolymers grafted via ATRP. [Figure 1]
• Lichtenberger, D. L., Talbot, M. O., Evans, D. H., & Glass, R. S. (2017, October). Electrocatalytic hydrogen production by bridged butterfly [2Fe-2S] catalysts. In Abstracts of Papers, 254th ACS National Meeting & Exposition, New Orleans, LA, United States, March 18-22, 2018.
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  Hydrogen gas is in high demand, primarily due to its role in the production of ammonia. Current means of hydrogen production involve carbon-rich sources which generate ten times the amount of carbon dioxide for hydrogen that is produced. Due to environmental concerns, alternative means of hydrogen production must be developed. The butterfly [2Fe-2S] active sites of diiron-hydrogenase enzymes efficiently reduce protons to hydrogen gas, prompting research into biologically inspired catalysts designed for hydrogen production. A series of bridging [2Fe-2S] catalysts were synthesized and characterized. Using weak acid as the proton source in an organic solvent, observed rate constants were measured to be greater than 1 x 106 s-1. Density functional theory was employed to compute reduction potentials, pKa values, and Gibbs free energies leading to the proposed catalytic mechanisms. Kinetic analysis comparisons were conducted between this series and the μ-1,2-benzenedithiolate system. Electrochemical simulations are consistent with the conclusions drawn.
• Talbot, M. O., Stratton, L. M., Evans, D. H., Glass, R. S., & Lichtenberger, D. L. (2017, 2017///). Fast electrocatalytic production of hydrogen by thiophenedithiolate bridged butterfly [2Fe-2S] clusters.. In Abstracts of Papers, 254th ACS National Meeting & Exposition, Washington, DC, USA, August 20-24, 2017, INOR-221.
• Lichtenberger, D. L., Clary, K. E., Marx, J. M., Evans, D. H., & Glass, R. S. (2016, 2016///). Electrocatalytic hydrogen production with [2Fe2S] clusters yielding high turnover frequencies and numbers.. In Abstracts, 68th Southeastern Regional Meeting of the American Chemical Society, Columbia, SC, United States, October 23-26, SERMACS-1004.

Presentations

• Lichtenberger, D. L., Brezinski, W., Evans, D. H., Glass, R. S., Karayilan, M., Matyjaszewski, K., Pyun, J., & Clary, K. (2018, June). Metallopolymers with [FeFe]-Hydrogenase Active Site Mimetics via ATRP. National Grad Research Polymer Conference.
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  The driving force for making metal-containing polymers or metallopolymers is to combine the useful properties of polymers and metal-complexes into a single material.1 In most cases, the metal-complex plays a pivotal role in the metallopolymers. FeFe-Hydrogenase, for example, is an enzyme which is composed of a metal-complex surrounded by a protein scaffold.2 This enzyme is responsible for generation of molecular hydrogen in an anaerobic bacteria. Extensive research has been done on mimicking the active site of this enzyme to generate a carbon-free energy carrier (i.e., H2) using earth-abundant metals (i.e., Fe and/or Ni).3 However, most of the enzyme mimics suffer from limited lifetime, low oxygen stability and low solubility all of which point out the importance of secondary coordination sphere around the active site. We demonstrate the synthesis of a difunctional metalloinitiator from a FeFe-H2ase mimic which is later used for synthesis of metallopolymers via a controlled radical polymerization technique, ATRP. By changing the pendant functionalities on the polymeric arms, we demonstrate the ability of tuning properties ( e.g., solubility) and enhancing catalytic activity of the resulting metallopolymers. Electrocatalytic activity of various metallopolymers and random metallocopolymers have been investigated in water at pH 7. The metallopolymers with amine functionality have shown substantially high H2 evolution catalytic activity and longer lifetime even in the presence of oxygen. We will discuss our most recent efforts on synthesis and applications of different metallopolymers for electrocatalytic hydrogen evolution reactions.
• Lichtenberger, D. L. (2015, Summer). Thermodynamics and kinetics of redox processes of hydrogenase mimics based on photoelectron spectroscopy, electrochemistry, and computations. Telluride Science Research Center Workshop "Structure and Function of the Hydrogenase Mimics". Telluride, CO.
• Lichtenberger, D. L. (2013, November). Moving Electrons for Sustainability Electrocatalytic Production of Molecular Hydrogen. University of Texas El Paso. University of Texas, El Paso.
• Lichtenberger, D. L. (2013, October). ACS on Campus. ACS on Campus. Tucson, AZ: The University of Arizona.
• Lichtenberger, D. L. (2012, August). [FeFe]-hydrogenase active site mimics for hydrogen production. National Nanotechnology Center (NANOTEC). Thailand Science Park, Pathum Thani, Thailand.
• Lichtenberger, D. L. (2012, August). [FeFe]-hydrogenase active site mimics for hydrogen production. Suranaree University of Technology. Nakhon Ratchasima, Thailand.

Poster Presentations

• Sill, S., Hall, G., Stratton, L., Glass, R., Lichtenberger, D., & Evans, D. (2014, March). [FeFe]-hydrogenase active site mimic with an annulated terthiophene group as an approach to a visible light absorbing photocatalyst for hydrogen production. 247th ACS National Meeting. Dallas, TX.
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  Dates: 03/16-03/20
• Chiarella, G., Cotton, A., Wilkinson, C., Durivage, J., Lichtenberger, D., & Murillo, C. (2013, November). Metal-cyclic guanidinate complexes: preparation of highly soluble closed-shell molecules with the lowest ionization energies. 69th Southwest Regional Meeting of the American Chemical Society. Waco, TX.
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  Dates: 11/16-11/19
• Enemark, J., Wiebelhaus, N., Stein, B., Kirk, M., & Lichtenberger, D. (2013, January). Spectroscopic and Theoretical Studies of Metal Dithiolenes Provide Insight into the Roles of Electron Occupancy and Molecular Distortions in the Reactivity of Molybdenum Enzymes. Metals in Biology Gordon Research Conference. Ventura, CA.
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  Dates: 01/20-01/25
• Sakamoto, T., Smith, E., Meyer, G., Swenson, M., Glass, R., Lichtenberger, D., Evans, D., Harb, M., & Weigand, W. (2013, April). Effects of increasing the diselenolato bridge linker chain length in dichalcogenolato [FeFe]-hydrogenase model compounds. 245th ACS National Meeting. New Orleans, LA.
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  Dates: 04/07-04/11
• Stratton, L., Hall, G., Evans, D., Glass, R., & Lichtenberger, D. (2013, April). Electrocatalytic pathways of [(?-3,4-thiophenedithiolato)][Fe(CO)3]2 compared to [(?-1,2-benzenedithiolato)][Fe(CO)3]2 for the catalytic production of H2. 245th ACS National Meeting. New Orleans, LA.
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  Dates: 04/07-04/11
• Borowski, S., Hall, G., Evans, D., Glass, R., & Lichtenberger, D. (2012, March). Electrocatalytic hydrogen production with 2-azapropane-1,3-dithiolato-bridged hydrogenase-mimic. 243rd ACS National Meeting & Exposition. San Diego, CA.
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  Dates: 03/25-03/29
• Enemark, J., Wiebelhaus, N., Cranswick, M., Klein, E., Lockett, L., & Lichtenberger, D. (2012, March). Metal-sulfur valence orbital interaction energies in metal-dithiolene complexes: Determination of charge and overlap interaction energies by comparison of core and valence ionization energy shifts. 243rd ACS National Meeting & Exposition. San Diego, CA.
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  Dates: 03/25-03/29
• Hall, G., Swenson, M., Triphahn, J., Ossowski, S., Chin, J., Okumura, N., Zakai, U., Evans, D., Glass, R., & Lichtenberger, D. (2012, March). Noninnocent quinonedithiolato ligands in [FeFe]-hydrogenase active site mimics: EPR characterization of spin density delocalization in the radical anions. 243rd ACS National Meeting & Exposition. San Diego, CA.
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  Dates: 03/25-03/29
• In-noi, O., Lichtenberger, D., Glass, R., Evans, D., Enemark, J., & Haller, K. (2012, March). Synthesis, structural, and electrochemical studies of diiron hydrogenase mimics: [(m-SR)(m-SR')Fe2(CO)6] system. 243rd ACS National Meeting & Exposition. San Diego, CA.
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  Dates: 03/25-03/29
• Lichtenberger, D. L. (2012, August). National Nanotechnology Center (NANOTEC). National Nanotechnology Center (NANOTEC). Pathum Thani, Thailand: Thailand Science Park.
• Lichtenberger, D. L. (2012, Fall). Suranaree University of Technology. Suranaree University of Technology. Nakhon Ratchasima, Thailand.
• Smith, E., Sakamoto, T., Evans, D., Glass, R., & Lichtenberger, D. (2012, March). Electrochemical and computational investigations of (µ-pyrazine-2,3-dithiolato)diironhexacarbonyl as a catalyst for proton reduction from weak acids. 243rd ACS National Meeting & Exposition. San Diego, CA.
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  Dates: 03/25-03/29
• Whelan, L., Hall, G., Nichol, G., Evans, D., Glass, R., & Lichtenberger, D. (2012, March). Electronically similar and electrochemically dissimilar [(m-2,4-pentanedithiolato)][Fe(CO)3]2 and [(m-1,3-propanedithiolato)][Fe(CO)3]2: A structural, electrochemical, and computational study of diiron hydrogenase active site mimics. 243rd ACS National Meeting & Exposition. San Diego, CA.
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  Dates: 03/25-03/29

Others

• Lichtenberger, D., & Murillo, C. (2015, 2015/03/19/). Preparation of tungsten complexes having low ionization energy.. U.S. Pat. Appl. Publ..
• Lichtenberger, D. L. (2014, August). Cover Art of Organometallics. Organometallics.
• Lichtenberger, D. L. (2014, February). JACS Image Challenge #255. J. Am. Chem. Soc..
• Lichtenberger, D. L. (2014, January). Cover Art of the Journal of the American Chemical Society. J. Am. Chem. Soc..
• Lichtenberger, D. L. (2013, November). Science Studio: The Hydrogen Economy. Radio Station KTEP, University of Texas, El Paso.