A COMPARATIVE FULLY RELATIVISTIC NONRELATIVISTIC 1ST-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-Sl ater calculations were also carried out for the M = Th complex. The no nrelativistic calculations indicate that metal-ligand interactions inv olving the highest energy ligand orbitals involve primarily metal 5f o rbitals while 6d admixtures are found for lower energy orbitals. The M -0 bonding is both sigma and pi in nature and involves primarily metal 6d atomic orbitals. Evidence of a charge redistribution mechanism alo ng the CH3 --> O --> M --> Cp3 direction provides a satisfactory expla nation for the shortened M-0 distances and strong propensity for nearl y linear M-O-CH3 linkages observed in diffraction studies. The fully r elativistic calculations show that metal d contributions are slightly underestimated at the nonrelativistic level. Such deviations do not, h owever, 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 nonrel ativistic level if optimized basis sets and potential representations are used.