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A nonclassical Pt···H−O−H hydrogen bond was observed in the hydration shell of cisplatin and its aquated derivatives, including the highly positively charged cis-[Pt(NH3)2(H2O)2]2+ complex. This weak interaction is stabilized by orbital and dispersion energies. At the same time the interaction has to be enabled by a fitting network of hydrogen bonds formed by neighboring solvent molecules and the ligands of the complex.
The structure of the hydration shell of cisplatin, cis-[Pt(NH3)2Cl2], and its aquated derivatives cis-[Pt(NH3)2Cl(H2O)]+, cis-[Pt(NH3)2OH(H2O)]+, and cis-[Pt(NH3)2(H2O)2]2+ were studied by a number of density functional molecular dynamics (DFT-MD) simulations (from 30 to 250 ps) in which Pt(II) complexes were immersed in a periodic box with 72 explicit water molecules. Furthermore, Pt(II) complex–water binding energy curves and full DFT optimizations of clusters derived from the lowest potential energy DFT-MD frames offered a deeper insight into the structure of the first hydration shell and electronic changes connected with the formation of a nonclassical Pt···H–O–H (Pt···Hw) hydrogen bond (inverse hydration). The probability of a Pt···Hw interaction decreases with increasing charge of the platinum complex due to disadvantageous electrostatics. The main stabilization comes from the charge transfer being followed by polarization and dispersion. Ligands form a framework for the network of H-bond interactions between the solvent molecules, which play an important role in the promotion/suppression of the formation of the Pt···Hw interactions. In the +2 charged diaqua complex the Pt···Hw interaction is still attractive but cannot compete with classical H bonds between solvent molecules. Thus, the formation of a Pt···Hw interaction is the result of a suitable solvent H-bonding network and the probability of its incidence decreases with increasing flexibility of the solvent.
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