Electronic structure and photoelectron spectroscopy of the monomeric uranium(III) alkyl [eta(5)-(CH3)(5)C-5]2UCH[Si(CH3)(3)](2) .13. Photoelectron spectroscopy of f-element organometallic complexes.

The ground state configuration and the electronic structure of the title compound have been investigated by relativistic effective core potential configuration interaction ab initio calculations and He I/He II UV photoelectron spectroscopy. It has been found that the (4)A '' [(a')(1) (a '')(1) (a')(1)] state, associated with the 5f(3) uranium configuration, represents the molecular ground state and is close in energy to the higher-lying (4)A '' (5f(2) 6d(1)) state. The comparison of the present photoelectron data and theoretical results with those reported for analogous lanthanide complexes indicates close similarities in metal-ligand bonding between the 4f and 5f M(III) organometallics. The relative stability of the 5f(3) versus 5f(2) 6d(1) uranium ion configuration correlates with the inherent chemical stability of U(III) organometallics, compared to that of the Th(III) (6d(1)) analogues. The easier M(IV) --> M(III) reduction for U(IV) relative to Th(IV), the numerous examples of reductive elimination processes in U(IV) organometallic chemistry, and the greater Th-R versus U-R experimental homolytic bond disruption energies in cyclopentadienylactinide(IV) alkyls can all be rationalized in terms of the differing electronic stabilities of the U(III) and Th(III) ground states.