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 D2d 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.