Metal-metal bonds involving the f elements. 4. Molecular orbital studies of metal-metal and metal-ligand interactions in dinuclear uranium(V) systems

The electronic structures of a series of dinuclear uranium(V) complexes have been investigated using X{alpha}-SW molecular orbital calculations including quasirelativistic corrections. Complexes of the formula U{sub 2}H{sub 10} and U{sub 2}(OH){sub 10} were used to model the metal-ligand {sigma} and {pi} interactions, respectively, in the known species U{sub 2}(O-i-Pr){sub 10}. Two basic geometries were investigated: a vertex-sharing bioctahedron with only terminal ligands (D{sub 4h} symmetry) and an edge-sharing bioctahedron containing two bridging ligands (D{sub 2h} symmetry). The latter geometry, which is that of U{sub 2}(O-i-Pr){sub 10}, was also examined at U-U bonding and nonbonding distances. The calculations indicate that the U-U interactions are significantly perturbed when H is replaced by OH, owing to strong donation from the OH p{pi} orbitals into selected U 5f orbitals. The result is a lack of any appreciable U-U interaction for U{sub 2}(OH){sub 10} in either the D{sub 4h} or D{sub 2h} geometry. In addition, the overall OH {pi} donation to the U 5f levels is enhanced in the D{sub 2h} geometry. The electronic structure of a hypothetical U(V) dimer, Cp{sub 2}U{sub 2}O{sub 4}, was also examined in both bridged and unsupported geometries. The unbridged geometry, like that for U{sub 2}(OH){sub 10}, suffered from a destabilization of the U-U {sigma} orbital due to ligand {pi} donation and revealed no net U-U bonding. However, the geometry exhibiting two bridging oxo ligands maintains the U-U {sigma}-bonding MO as its lowest energy U 5f orbital. 21 refs., 8 figs., 8 tabs.