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.

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The electronic structures of CpRe(NO)(L)R and Cp*Re(NO)(L)R (Cp = η5-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.