The role of solvent charge donation in the stabilization of metal ions in aqueous solution

We analyze charge density transfer from water to solvated transition metal (TM) ions in different formal oxidation states (FOSs) in aqueous solution by first principles and relate the degree of stabilization of the solvated cations to the charge donation from the water ligands. We find remarkable charge stability on the metal center regardless of FOSs. This effect is similar to what has previously been shown for charges on TM cations in inorganic crystals. This ligand-to-metal charge transfer results in softening of the ligand O-H bonds, which can be used to explain the formation of higher-FOS transition metalates and oxycations.