The role of solvent charge donation in the stabilization of metal ions in aqueous solution
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esearch Letter
The role of solvent charge donation in the stabilization of metal ions in aqueous solution Daniel Koch and Sergei Manzhos, Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore Address all correspondence to Sergei Manzhos at [email protected] (Received 1 July 2018; accepted 6 August 2018)
Abstract 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.
Introduction Reduction and oxidation of hydrated metal cations in aqueous solution are the archetype of half-cell reactions, and redox processes involving two metal species in aqueous solution (and in bulk) are arguably the most prominent examples of cell reactions in electrochemistry. Metal–ligand interactions, like the ones occurring between the central ion and the solvation shell in aqueous metal salt solutions, have been and still are a very active area of research.[1–5] Especially coordinated transition metals (TMs) play a major role in numerous applications, such as catalysis or energy materials.[6–8] Recent investigations shed light on the charge density distribution in TM compounds, revealing a surprisingly stable charge remainder localized on the metal cations, enabled by increasing TM d orbital contributions to the frontier orbitals with an increasing formal oxidation state (FOS) of the metal.[9,10] We have argued in a previous work that the FOS formalism based on electron counting[11–13] falls short of the description of actual charge density distributions in real compounds and that the mechanistic understanding of redox processes in applications should consider actual charge distributions and fluxes.[14,15] The understanding of charge self-regulation in solid TM complexes which leads to remarkably stable charges (i.e., actual oxidation states) on the metal center which are far from those implied by the FOS is helped by the fact that shortrange charge transfer need not be quantized. Redox reactions in solutions involve long-range charge transfer which is quantized. This, however, does not exclude the charge selfregulation which must in this case operate between the metal center and the solvation shell.
In this work, we investigate the charge states of TMs in metal aquo complexes, using the quantum theory of atoms in molecules (QTAIM), an established approach to describe charge density distributions in molecular and periodic systems.[16] We have shown in Ref. [14] that QTAIM reliably accounts for charge
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