A Systematic ab Initio Study of the Hydration of Selected Palladium Square-Planar Complexes. A Comparison with Platinum Analogues

The hydration surface of four palladium square-planar complexes (neutral cis -dichlorodiamminepalladium PdCl 2 (NH 3 ) 2, neutral trans -dichlorodiamminepalladium PdCl 2 (NH 3 ) 2, tetraamminepalladium cation [Pd(NH 3 ) 4 ] 2+, and tetrachloropalladium anion [PdCl 4 ] 2- ) has been examined by advanced quantum-chemical calculations. Preliminary geometry optimizations were carried out using the second-order Møller−Plesset level of theory with frozen core approximation, utilizing the 6-31G* basis set for H, N, O, and Cl atoms. Pd was described with the Stuttgart relativistic pseudopotential with a basis set of corresponding quality for the explicitly treated electrons. Final reoptimization of all the species considered in the hydration scheme was done at the MP2 (full) level. Then, the reaction surfaces of the structures localized by optimizations were constructed utilizing the MP4 single-point evaluations with additional inclusion of diffuse functions. The computed results were compared with corresponding data of analogous platinum complexes. The Pd and Pt energy surfaces resemble each other to a surprisingly large extent. Practically all qualitative trends, such as cis/trans energy ordering, are identical, and the solvation energies of Pd and Pt species differ only by a few (at most 10) kcal/mol. Concerning the markedly different biochemical and pharmacological roles of Pt- and Pd-based compounds, our basic conclusion is that the difference between cisplatin and analogous palladium complexes cannot be rationalized considering the energetics (thermodynamic properties) of hydration because these properties do not differ significantly.