DFT study of uranyl peroxo complexes with H₂O, F^-, OH^-, CO₃^2-, and NO₃^-

The structural and electronic properties of monoperoxo and diperoxo uranyl complexes with aquo, fluoride, hydroxo, carbonate, and nitrate ligands have been studied using scalar relativistic density functional theory (DFT). Only the complexes in which the peroxo ligands are coordinated to the uranyl moiety in a bidentate mode were considered. The calculated binding energies confirm that the affinity of the peroxo ligand for the uranyl group far exceeds that of the F^-, OH^-, CO₃^2-, NO₃ ^-, and H₂O ligands. The formation of the monoperoxo complexes from UO₂(H₂O)₅²⁺ and HO₂^- were found to be exothermic in solution. In contrast, the formation of the monouranyl-diperoxo, UO₂(O₂) ₂X₂^4- or UO₂(O₂) ₂X^4-/3- (where X is any of F^-, OH^-, CO₃^2-, or NO₃^-), complexes were all found to be endothermic in aqueous solution. This suggests that the monoperoxo species are the terminal monouranyl peroxo complexes in solution, in agreement with recent experimental work. Overall, we find that the properties of the uranyl-peroxo complexes conform to well-known trends: the coordination of the peroxo ligand weakens the U-O_yl bonds, stabilizes the σ(d) orbitals and causes a mixing between the uranyl π- and peroxo σ- and π-orbitals. The weakening of the U-O_yl bonds upon peroxide coordination results in uranyl stretching vibrational frequencies that are much lower than those obtained after the coordination of carbonato or hydroxo ligands.