A DFT study of uranyl hydroxyl complexes: structure and stability of trimers and tetramers

A DFT study of U(VI) hydroxy complexes was performed with special attention paid to the [(UO2)3(OH)5(H2O)4-7]+ and [(UO2)4(OH)7(H2O)5-8]+ species. It was established that the ionicity of the U=O bond increased when moving from [(UO2)(H2O)5]2+, [(UO2)2(OH)(H2O)8]3+, [(UO2)2(OH)2(H2O)6]2+, [(UO2)3(OH)5(H2O)4-6]+ to [(UO2)4(OH)7(H2O)5-8]+ species. In both [(UO2)3(OH)5(H2O)4-6]+ and [(UO2)4(OH)7(H2O)5-8]+ complexes, the U=O bond was observed to have a range of different lengths which depended on the composition of the first coordination sphere of UO22+. The cyclic structures of trimeric complexes were somewhat more stable than their linear structures, which was probably due to the steric effect.