Electronic structure and photoelectron spectra of bispentalene complexes of thorium and uranium

The preparation of [U{η8-C8H4(1,4-SiiPr3)2}2] is reported. The binding of the pentalene ligands in an η8 mode is examined by density functional calculations and photoelectron spectroscopy. Geometry optimization of [M(η8-C8H6)2], M = Th with D2d and D2 symmetry constraints, gives structures in good agreement with the X-ray structure found for [Th{η8-C8H4(1,4-SiiPr3)2}2]; in particular the folded nature of the ligand is well reproduced by the calculation. Examination of the barrier to relative rotation of the two ligands only showed a significant energy rise when the two rings were eclipsed. Geometry optimization for M = U, assuming D2d symmetry and a triplet state, gave a structure similar to the Th compound but with shorter metal-carbon bond lengths. The two U f electrons were constrained to occupy degenerate orbitals of e symmetry. He I and He II spectra of [M{η8-C8H4(1,4-SiiPr3)2}2], M = Th and U, are presented and assigned with the aid of density functional calculations. The principal binding is by δ bonds between the upper occupied π orbitals of the pentalene dianion and the actinide d and f orbitals of appropriate symmetry