Theoretical investigations of uranyl-ligand bonding: Four- and five-coordinate uranyl cyanide, isocyanide, carbonyl, and hydroxide complexes

The coordination and bonding of equatorial hydroxide, carbonyl, cyanide (CN-), and isocyanide (NC-) ligands with uranyl dication, [UO2](2+), has been studied using density functional theory with relativistic effective core potentials. Good agreement is seen between experimental and calculated geometries of [UO2(OH)(4)](2-). Newly predicted ground-state structures of [UO2(OH)(5)](3-), [UO2(CO)(4)](2+), [UO2(CO)(5)](2+), [UO2(CN)(4)](2-), [UO2(CN)(5)](3-), [UO2(NC)(4)](2-), and [UO2(NC)(5)](3-) are reported. Four-coordinate uranyl isocyanide complexes are the predicted gas-phase species while five-coordinate uranyl cyanide complexes are energetically favorable in aqueous solution. Small energy differences between cyanide and isocyanide complexes indicate the energetic feasibility of mixed cyanide and isocyanide complexes. A D-2d uranyl tetrahydroxide is the dominant gas-phase and aqueous species, but formation of uranyl carbonyl complexes is seen to be exothermic in the gas-phase and endothermic in aqueous solution.