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.