Theoretical studies of An(II)(2)(C(8)H(8))(2) (An = Th, Pa, U, and Np) complexes: the search for double-stuffed actinide metallocenes.

Complexes of the form An(2)(C(8)H(8))(2) (An = Th, Pa, U, and Np) were investigated using density functional theory with scalar-relativistic effective core potentials. For uranium, a coaxial isomer with D(8h) symmetry is found to be more stable than a C(s) isomer in which the dimetal unit is perpendicular to the C(8) ring axis. Similar coaxial structures are predicted for Pa(2)(C(8)H(8))(2) and Np(2)(C(8)H(8))(2), while in Th(2)(C(8)H(8))(2), the C(8)H(8) rings tilt away from the An-An axis. Going from Th(2)(C(8)H(8))(2) to Np(2)(C(8)H(8))(2), the An-An bond length decreases from 2.81 A to 2.19 A and the An-An stretching frequency increases from 249 to 354 cm(-1). This is a result of electrons populating An-An 5f pi- and delta-type bonding orbitals and varphi nonbonding orbitals, thereby increasing in An-An bond order. U(2)(C(8)H(8))(2) is stable with respect to dissociation into U(C(8)H(8)) monomers. Disproportionation of U(2)(C(8)H(8))(2) into uranocene and the U atom is endothermic but is slightly exothermic for uranocene plus (1)/(2)U(2), suggesting that it might be possible to prepare double stuffed uranocene if suitable conditions can be found to avoid disproportionation.