Vibrational Progressions in the Valence Ionizations of Transition Metal Hydrides: Evaluation of Metal-Hydride Bonding and Vibrations in (η 5 -C 5 R 5 )Re(NO)(CO)H [R ) H, CH 3 ] Dennis L. Lichtenberger,* ,† Nadine E. Gruhn, † Anjana Rai-Chaudhuri, † Sharon K. Renshaw, † John A. Gladysz,* ,‡,§ Haijun Jiao, § Jeff Seyler, ‡ and Alain Igau ‡ Contribution from the Center for Gas-Phase Electron Spectroscopy, Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721, Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112, and Institut fu ¨ r Organische Chemie, Friedrich-Alexander UniVersita ¨ t Erlangen-Nu ¨ rnberg, Henkestrasse 42, 91054 Erlangen, Germany Received September 10, 2001 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 ) η 5 -C5H5, Cp* ) η 5 -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 H 2 upon ionization. Removal of an electron from the Re-H or Re-D bonds leads to a quantum-mechanical inner sphere reorganization energy (λ QM ) 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. Introduction The nature of the metal-hydride bond is important to many catalytic, synthetic, and materials processes. Experimentally, photoelectron spectroscopy has proven to be an invaluable tool for obtaining information related to the energetics and nature of metal-ligand bonding in many inorganic and organometallic molecules. 1-6 In addition to providing ionization energies that are important for understanding the valence electronic structure of a molecule, photoelectron spectroscopy reveals vibrational and structural information for the ground and excited positive ion states of the molecule when one or more vibrational progressions are resolved in the ionizations. First-order inter- pretation of vibrational structure in a photoelectron ionization is straightforward. 7 In the orbital model of electronic structure, a particular positive ion state is obtained from the neutral molecule by removal of an electron from an occupied orbital. Comparison of the vibrational and structural features of the positive ion state to those of the neutral molecule indicates the bonding nature of an electron in that orbital. Qualitatively, if the orbital from which the electron is removed is essentially nonbonding and no geometry change occurs with that ionization, then the most intense ionization will be to the lowest vibrational level in the positive ion. Ionizations to higher excited vibrational quantum levels in the positive ion gain intensity as the minimum-energy structure of the positive ion state becomes increasingly different from that of the neutral molecule. Most often, the geometry change is associated with a bond distance in the molecule. Quantitatively, the difference between the vibrational frequency of the positive ion and the vibrational frequency of the neutral molecule reflects the change in the vibrational force constant caused by removal of the orbital electron, and analysis of the intensity pattern of the vibrational progression measures the change in bond distance caused by removal of the orbital electron. * To whom correspondence should be addressed. † University of Arizona. ‡ University of Utah. § Friedrich-Alexander Universita ¨t Erlangen-Nu ¨rnberg. (1) Green, J. C. Struct. Bonding (Berlin) 1981, 43, 37-112. (2) Solomon, E. I. Comments Inorg. Chem. 1984, 3, 225-320. (3) Green, J. C. Acc. Chem. Res. 1994, 27, 131-137. (4) Lichtenberger, D. L.; Gruhn, N. E.; Renshaw, S. K. J. Mol. Struct. 1997, 405, 79-86. (5) Li, X.; Bancroft, G. M.; Puddephatt, R. J. Acc. Chem. Res. 1997, 30, 213- 218. (6) Wu, J.; Bancroft, G. M.; Puddephatt, R. J.; Hu, Y. F.; Li, X.; Tan, K. H. Inorg. Chem. 1999, 38, 4688-4695. (7) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Wiley-Interscience: New York, 1970. Published on Web 01/22/2002 10.1021/ja0120227 CCC: $22.00 © 2002 American Chemical Society J. AM. CHEM. SOC. 9 VOL. 124, NO. 7, 2002 1417