Photoelectron Spectroscopy of f-Element Organometallic Complexes. 6. Electronic Structure of Tetrakis(cyclopentadienyl)actinide Complexes

A description of the electronic structure of M(η5-C5H4CH3)4 (M = Th, U) complexes as based upon He I and He II photoelectron spectra and relativistic Xα-SW calculations is presented. Despite the true S4 symmetry of the molecules, the outermost MO's are grouped as if the complexes have a pure tetrahedral ligation environment. As a consequence, donation to empty metal orbitals appears to be highly dominated by the symmetry requirements. The HOMO in Cp'4U is almost pure 5f in character, and the corresponding 2F5/2 final state produced upon ionization is split by operation of a Td ligand field. The relative intensity variations observed on passing from He I to He II spectra are quite unexpected. It appears that the He II increases of PE signals due to MO's having large metal 6d character are greater than those due to MO's having comparable 5f admixture.