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]

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 (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.