Photoelectron spectroscopy of f-element organometallic complexes. 9. A comparative fully relativistic/nonrelativistic first-principles X.alpha.-DVM and photoelectron spectroscopic investigation of electronic structure in homologous 4f and 5f tris(.eta.5-cyclopentadienyl)metal(IV) alkoxide complexes

The electronic structure of (eta5-C5H5)3MOR (M = Ce, Th, U) complexes has been investigated by He I and He II UV photoelectron spectroscopy combined with SCF Xalpha-DVM calculations. Fully relativistic Dirac-Slater calculations were also carried out for the M = Th complex. The nonrelativistic calculations indicate that metal-ligand interactions involving the highest energy ligand orbitals involve primarily metal 5f orbitals while 6d admixtures are found for lower energy orbitals. The M-O bonding is both sigma and pi in nature and involves primarily metal 6d atomic orbitals. Evidence of a charge redistribution mechanism along the CH3 --> O --> M --> Cp3 direction provides a satisfactory explanation for the shortened M-O distances and strong propensity for nearly linear M-O-CH3 linkages observed in diffraction studies. The fully relativistic calculations show that metal d contributions are slightly underestimated at the nonrelativistic level. Such deviations do not, however, alter the overall description of the metal-ligand bonding. The nonrelativistic configuration of the metal center compares well with the relativistic data. Gas-phase ionization energies can be accurately and comparably evaluated at the computationally more efficient nonrelativistic level if optimized basis sets and potential representations are used.