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 UO2(2+). The cyclic structures of trimeric complexes were somewhat more stable than their linear structures, which was probably due to the steric effect.