DFT study of oxo-functionalized pentavalent dioxouranium complexes: Structure, bonding, ligand exchange, dimerization, and U(V)/U(IV) reduction of OUOH and OUOSiH₃ complexes

The structural and electronic properties of model oxo-functionalized pentavalent dioxouranium complexes have been studied using scalar relativistic density functional theory (DFT) calculations. The electronic structures of these complexes are compared to those of their hexavalent and pentavalent counterparts with free axial oxo groups while paying particular emphasis on the effect of oxo-functionalization on the formation of binuclear complexes, the U(V)/U(IV) redox potentials, as well as ligand exchange between the axial and equatorial regions of the dioxouranium moiety. The stabilization of the σ(d) orbitals of the UO₂ moiety is one of the major effects of oxo-functionalization. The origin of this effect is the mixing of the σ(d) orbital of the uranyl group with the σ(OH)/σ(OSiH₃) orbitals of the axial OH/OSiH₃ group. The 6p atomic orbitals of the uranium center are mixed to a greater extent with the σ(d) orbital after stabilization caused by oxo-functionalization. The asymmetric nature of the oxo-functionalization has dramatic effects not only on the U-O bond lengths (elongation by up to 0.24 Å) and U-O bond orders (loss of a full bond order) but also on the formation and type of U₂O₄ core found in binuclear complexes. The loss of a full bond order upon oxo-functionalization means the axial U-OH/U-OSiH₃ bonds are only slightly stronger than they would be if they were found in the equatorial region of the uranyl moiety. This raises the possibility of ligand exchange between the axial and equatorial regions as well as increasing the stability of the binuclear complexes with butterfly-shaped U₂O₄ cores relative to those with diamond U₂O₄ cores. Reductive oxo-functionalization results in complexes with lower electron density at their U(V) centers in comparison to UO₂⁺ complexes. This has dramatic effects on the calculated U(V)/U(IV) redox potentials.