How do the electronic structures of low-symmetry metal-hydride and -alkyl complexes compare? Photoelectron spectroscopy and computational studies of (η⁵-C₅R₅)Re(NO)(L)R′(L = CO, P(C₆H₅)₃; R, R′ = H, CH₃)

The electronic structures of CpRe(NO)(L)R and Cp*Re(NO)(L)R (Cp = *⁵-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″ p_π 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.