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. T he binding of the pentalene ligand in an eta(8) mode is examined by density functional calculations and photoelectron spectroscopy. Geometry optimizat ion 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{e ta(8)-C8H4(1,4-(SiPr3)-Pr-i)(2)}(2)]; in particular the folded nature of th e ligand is well reproduced by the calculation. Examination of the barrier to relative rotation of the two ligands only showed a significant energy ri se when the two rings were eclipsed. Geometry optimization for M = U, assum ing D-2d symmetry and a triplet state, gave a structure similar to the Th c ompound but with shorter metal-carbon bond lengths. The two U f electrons w ere 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 pre sented and assigned with the aid of density functional calculations. The pr incipal binding is by delta bonds between the upper occupied pi orbitals of the pentalene dianion and the actinide d and f orbitals of appropriate sym metry.