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

The preparation of [UC{eta(8)-C8H4(1,4-(SiPr3)-Pr-i)(2)}(2)] is reported. The binding of the pentalene ligand in an eta(8) mode is examined by density functional calculations and photoelectron spectroscopy. Geometry optimization of [M(eta(8)-C8H6)(2)], M = Th with D-2d and D-2 symmetry constraints, gives structures in good agreement with the X-ray structure found for [Th{eta(8)-C8H4(1,4-(SiPr3)-Pr-i)(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 D-2d 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{eta(8)-C8H4(1,4-(SiPr3)-Pr-i)(2)}(2)], M = Th and U, are presented and assigned with the aid of density functional calculations. The principal binding is by delta bonds between the upper occupied pi orbitals of the pentalene dianion and the actinide d and f orbitals of appropriate symmetry.