How do the electronic structures of low-symmetry metal-hydride and -alkyl complexes compare? Photoelectron spectroscopy and computational studies of (eta(5)-C5R5)Re(NO)(L)R ' (L = CO, P(C6H5)(3); R, R ' = H, CH3)
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abstract
The electronic structures of CpRe(NO)(L)R and Cp*Re(NO)(L)R (Cp = *5-C5H5, Cp* = 5-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 bond orbitals, and the predominantly Cp e1 p orbitals are clearly observed. Comparison of the shapes and energies of the Cp and (Re-R) ionizations indicates an additional direct interaction between these orbitals that is sensitive to energy matching. This interaction results in a more delocalized -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 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 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 interaction with the R ligand. This difference in the two metal orbital distributions, along with the differences in energy, influences the rotational orientation of ligands at this site. The extent of this interaction is sensitive to the electron richness at the metal center.
