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.