Understanding the Hydration Structure of Square-Planar Aquaions: The [Pd(H2O)4]2+ Case
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Abstract
Ion solvation of monatomic cations has usually been rationalized on the basis of concentric shell models because of the electrostatic field generated by the metal cation. This work examines, by means of molecular dynamics simulations, the solvation phenomenon for a square-planar hydrate, [Pd(H2O)4]2+, addressing the question of the structure adopted by water solvent in the regions above and below the molecular plane. Specific ab initio intermolecular potentials describing the interaction between the ion and the solvent have been developed, extending the statistical implementation of the hydrated ion concept. Results show how water molecules in these regions present structural and dynamic properties markedly different from those of the first and the second shells. Whereas average distances are close to those of the first hydration shell, their orientation is similar to that of the second shell, and their mean residence times are even shorter than those of the second hydration shell. This region, called the meso shell, could help in understanding peculiar properties of transition-metal cation square-planar complexes, such as their particular facility to be incorporated in confined regions such as those present in complex biomolecules or interlayered solid structures.
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