Hydration of selected platinum complexes [PtCl42−, Pt(NH3)42+, and cis- and trans-platin–PtCl2(NH3)2] have been studied. Up to two solvent molecules have been considered to replace the ligands. In order to be able to draw conclusions about pH changes in the course of the hydration process, both H2O and OH− species were considered in the solvating process. The modified Gaussian 3 theory was adapted for the pseudopotential treatment of platinum complexes. Since a heavy element was present in the complexes, an additional stabilization due to the spin–orbit coupling and core-polarization potentials have been evaluated above the scheme of a G3 treatment. This spin–orbit coupling stabilization amounts to 2–5 kcal/mol but does not qualitatively change the hydration preferences. In accord with the experiment, neutral Pt(NH3)2(OH)2 was found to be the most stable complex for hydration of both cis- and trans-platin.

The thermodynamic and kinetic aspects of hydration reactions of cis-/transplatin were explored. The polarizable continuum model was used for estimation of solvent effects. Using the B3LYP/6-31+G(d) method, the structures were optimized and vibrational frequencies estimated. Interaction energies and activation barriers were determined at the CCSD(T)/6-31++G(d,p) level within the COSMO approach. An associative mechanism was assumed with a trigonal-bipyramidal structure of the transition state. Within the applied model, all the hydration reactions are slightly endothermic. The Gibbs energies of cisplatin hydration amount to 7.0 and 14.2 kcal/mol for the chloride and ammonium replacement, respectively. Analogous values for the transplatin reactions are 6.8 and 11.9 kcal/mol. The determined rate constants are by several (three to four) orders of magnitude larger for the dechlorination process than for deammination. The cisplatin dechlorination rate constant was established as 1.3 x 10(-4) s(-1) in excellent accord with the experiment.

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In the present work, an ab initio study on hydration (a metal-ligand replacement by water molecule or OH- group) of cis- and transplatin and their palladium analogs was performed within a neutral pseudomolecule approach (e.g., metal-complex+water as reactant complex). Subsequent replacement of the second ligand was considered. Optimizations were performed at the MP2/6-31+G(d) level with single-point energy evaluation using the CCSD(T)/6-31++G(d,p) approach. For the obtained structures of reactants, transition states (TS's), and products, both thermodynamic (reaction energies and Gibbs energies) and kinetic (rate constants) characteristics were estimated. It was found that all the hydration processes are mildly endothermic reactions-in the first step they require 8.7 and 10.2 kcal/mol for ammonium and chloride replacement in cisplatin and 13.8 and 17.8 kcal/mol in the transplatin case, respectively. Corresponding energies for cispalladium amount to 5.2 and 9.8 kcal/mol, and 11.0 and 17.7 kcal/mol for transpalladium. Based on vibrational analyses at MP2/6-31+G(d) level, transition state theory rate constants were computed for all the hydration reactions. A qualitative agreement between the predicted and known experimental data was achieved. It was also found that the close similarities in reaction thermodynamics of both Pd(II) and Pt(II) complexes (average difference for all the hydration reactions are approximately 1.8 kcal/mol) do not correspond to the TS characteristics. The TS energies for examined Pd(II) complexes are about 9.7 kcal/mol lower in comparison with the Pt analogs. This leads to 10(6) times faster reaction course in the Pd cases. This is by 1 or 2 orders of magnitude more than the results based on experimental measurements.

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.