Explaining Asymmetric Solvation of Pt(II) versus Pd(II) in Aqueous Solution Revealed by Ab Initio Molecular Dynamics Simulations.

The solvation behavior of Pt(II) versus Pd(II) has been studied in ambient water using ab initio molecular dynamics. Beyond the well-defined square-planar first solvation shell encompassing four tightly bonded water molecules as predicted by ligand field theory, a second coordination shell containing about 10 H2O is found in the equatorial region. Additional solvation in the axial regions is observed for both metals which is demonstrated to be induced by the condensed phase. For the Pt(II) aqua complex, however, this water molecule is bonded with one of its hydrogen atoms toward the cation, thus establishing a typical anionic solvation pattern, which is traced back to the electronic structure of Pt(2+) versus Pd(2+) cations, in particular to the anisotropic polarizability of their tetrahydrates. Systematic model calculations based on suitable aqua complex fragments embedded in a polarizable continuum solvent support the idea that anionic hydration is facilitated by the liquid. Furthermore, transient protolysis of water molecules in the first shell is observed for both divalent transition metal cations, being more pronounced for Pt(II) versus Pd(II). The relevance of these solvation features is discussed with respect to the different acidity of Pt(2+) versus Pd(2+) aqua ions in water, their different water ligand exchange rates, and force field modeling approaches.