Theoretical Studies of An2II(C8H8)2 (An = Th, Pa, U, and Np) Complexes: The Search for Double-Stuffed Actinide Metallocenes

Complexes of the form An(2)(C8H8)(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(C8H8)(2) and Np-2(C8H8)(2), while in Th-2(C8H8)(2), the C8H8 rings tilt away from the An An axis. Going from Th-2(C8H8)(2) to Np-2(C8H8)(2), the An An bond length decreases from 2.81 angstrom to 2.19 angstrom 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 phi nonbonding orbitals, thereby increasing in An An bond order. U-2(C8H8)(2) is stable with respect to dissociation into U(C8H8) monomers. Disproportionation of U-2(C8H8)(2) into uranocene and the U atom is endothermic but is slightly exothermic for uranocene plus 1/2U(2), suggesting that it might be possible to prepare double stuffed uranocene if suitable conditions can be found to avoid disproportionation.