An investigation of the electronic structure of some tris(η5-cyclopentadienyl)thorium(IV) and -uranium(IV) complexes by relativistic effective core potential ab initio calculations and gas-phase UV photoelectron spectroscopy

Comparative relativistic effective core potential ab initio calculations for both Th(IV) and U(IV) Cp3AnL (Cp = eta5-C5H5; L = CH3, BH4) Complexes are reported. The Cp-An bonding appears to be dominated by metal 6d orbitals interacting with ligand pi2 orbitals. Metal 5f orbitals provide a smaller contribution but are crucial for stabilization of the CP3An cluster. The stability of the An-CH3 bonding depends upon interactions involving metal 6d(z2)-based orbitals directed along the An-CH3 Vector. The L = BH4 ligand interactions are mediated by d(xz) and d(yz) atomic orbitals, which are even better suited for favorable overlap and, hence, for greater metal-ligand pi covalency. Ground 3A2 states have been found to be the most stable for the U(IV) complexes. The experimental He I/He II photoelectron data are consistent with the quantum chemical calculations and indicate a close similarity between ground-state properties of the present Th(IV) and U(IV) complexes.