Vibrational progressions in the valence ionizations of transition metal hydrides: evaluation of metal-hydride bonding and vibrations in (eta(5)-C(5)R(5))Re(NO)(CO)H [R = H, CH(3)].
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abstract
The first examples of vibrational structure in metal-ligand sigma-bond ionizations are observed in the gas-phase photoelectron spectra of CpRe(NO)(CO)H and CpRe(NO)(CO)H [Cp = eta(5)-C(5)H(5), Cp = eta(5)-C(5)(CH(3))(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 sigma-bonding orbitals. A vibrational progression is also observed in the corresponding ionization of the deuterium analogue, CpRe(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) A when an electron is removed from the Re-H or Re-D sigma-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 (lambda(QM)) of 0.34(1) eV. These observations suggest that even in these low symmetry molecules the orbital corresponding to the Re-H sigma 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.
