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 Xα-SW molecular orbital calculations including quasirelativistic corrections. Complexes of the formula U2H10 and U2(OH)10 were used to model the metal-ligand σ and π interactions, respectively, in the known species U2(O-i-Pr)10. Two basic geometries were investigated: a vertex-sharing bioctahedron with only terminal ligands (D4h symmetry) and an edge-sharing bioctahedron containing two bridging ligands (D2h symmetry). The latter geometry, which is that of U2(O-i-Pr)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 pπ orbitals into selected U 5f orbitals. The result is a lack of any appreciable U-U interaction for U2(OH)10 in either the D4h or D2h geometry. In addition, the overall OH π donation to the U 5f levels is enhanced in the D2h geometry. The electronic structure of a hypothetical U(V) dimer, Cp2U2O4, was also examined in both bridged and unsupported geometries. The unbridged geometry, like that for U2(OH)10, suffered from a destabilization of the U-U σ orbital due to ligand π donation and revealed no net U-U bonding. However, the geometry exhibiting two bridging oxo ligands maintains the U-U σ-bonding MO as its lowest energy U 5f orbital. 21 refs., 8 figs., 8 tabs