Square planar or octahedral after all? The indistinct solvation of platinum(II)
The solvation structures of Pd(II) and Pt(II) are typically reduced to the well-known square-planar structural motif, although it has been shown, in both experimental and theoretical investigations, that these solutes have a tendency to bind ligand molecules at elongated distance in axial coordination sites. In aqueous solution both ions form a tetragonally elongated octahedral first hydration shell (coordination number 6). However, a recent QM/MM MD simulation of Pd(II) in ammonia solution indicated that the properties of the axial ligands are highly sensitive with respect to the nature of the solvent. In the present study the solvation properties of these solutes in aqueous solution and pure ammonia are compared, presenting novel QM/MM MD data for Pt(II) in ammonia solution. The simulation results indicate that in the latter case the coordination sites in axial position remain unoccupied, thus indeed forming the prototype four-fold, square planar solvation structure (coordination number 4). The respective solvation properties are thoroughly evaluated and compared to experimental enthalpy and entropy of activation for the first shell ligand exchange, which similarly point towards the unique solvation of Pt(II) in ammonia solution. The observed solvation can be explained via the electrostatic potential of tetraammineplatinum(II) and -palladium(II), clearly highlighting a strongly negative potential perpendicular to the plane of solvation in the Pt(II) case. This result can be well interpreted in the rather simple context of Pearson's electronegativity and hardness, which had to be re-evaluated for Pt(II) employing novel data for the second and third ionisation potentials of elemental Pt. In contrast to earlier work these results indicate that the latter is less electronegative and softer than its Pd(II) counterpart. Together with the lower electronegativity and hardness of NH3 ligands compared to H2O the interaction between the solute and axial ligands is weakened up to a point at which the formation of a tetragonally elongated coordination is prevented.