EPR AND ANGLE-SELECTED ENDOR STUDY OF 5F-LIGAND INTERACTIONS IN THE [U(ETA(7)-C7H7)(2)](-) ANION, AN F(1) ANALOG OF URANOCENE

A frozen solution of [U(eta(7)-C7H7)(2)](-) is Studied by EPR and angl e-selected ENDOR spectroscopy. EPR confirms the anticipated f(1) confi guration of this compound. Analysis of the g tensor indicates that the ground state molecular orbital is principally made of 5f(pi) (51%) an d 5f(sigma) (39%) uranium orbitals. This composition results from the combined effect of the spin-orbit interaction and the strong f(delta)- ligand interactions. The small admixture of 5f(phi) (9%) uranium orbit als could result from a low-symmetry effect imposed by the solid-state packing forces at low temperature. Starting from the ground-state orb ital, the f-level ordering: consistent with the g tensor is found to b e (f(pi),f(sigma)) < f(phi) < f(pi) < f(phi), (f(sigma),f(pi)) < f(del ta) (antibonding). Proton ENDOR spectroscopy shows that there is a pos itive spin density p(pi) greater than or equal to 4 x 10(-2) in 2p(pi) carbon orbitals. which manifests the covalent character of the ground -state orbital.