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 2 d 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.