Structure and Bond Nature of the UF(5) Monomer

The structure and bond nature for the ground state of the UF(5) monomer have been investigated by means of relativistic and nonrelativistic discrete-variational (DV) Xalpha molecular orbital (MO) methods. It is found that the nonrelativistic calculation provides only less than half of the U-F bond overlap population obtained by the relativistic calculation for the monomer with the C(4)(v)() geometry which was determined experimentally. Comparison of the U-F bond overlap population and valence level structures between the C(4)(v)() and D(3)(h)() geometries indicates that the stability of the D(3)(h)() monomer is almost equal to that of the C(4)(v)() one. Furthermore, it is confirmed that an intermediate structure (C(2)(v)()) between the two geometries shows similar values of effective charge, orbital, and bond overlap populations. This suggests that the energy barrier between the D(3h) and C(4)(v)() geometries is very small and the geometry of the UF(5) monomer is fluxional between the C(4)(v)() and D(3)(h)() symmetries. This is consistent with the previous results reported by the Hartree-Fock method with relativistic effective core potentials [Wadt, W. R.; Hay, P. J. J. Am. Chem. Soc. 1979, 101, 5198]. The flexibility of the UF(5) structure originates from the fact that the U 5f atomic orbitals, which play a major role in the U-F bonding interactions, spread with wide angular distributions of the electrons in hybridization with the U 6d atomic orbital.