Ligand Exchange in Pd(II)-NaCl-H2O and Pd(II)-HCl-H2O Systems: Quantum-Chemical Consideration

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The behavior of potassium tetrachloropalladate(II) in media simulating biological liquids is studied. The rate of aquation in aqueous NaCl solutions is shown to be higher than the rate at which the Cl− ligand enters the inner coordination sphere of the Pd atom. In HCl solutions, the formation of the Pd chloro complexes predominates due to protonation of water molecules in the composition of aqua complexes. The reactions of replacement of the ligands (H2O molecules and H3O+ ion) in the planar Pd(II) complexes by the chloride ion are studied by the ZINDO/1 method. All the complexes containing H2O and H3O+ ligands, except for [Pd(H2O)4]2+, contain intramolecular hydrogen bonds. The ZINDO/1 and RHF/STO-6G(d) calculations revealed “nonclassic” symmetrical O⋯ H⋯O hydrogen bond in the [[Pd(H2O)3(H3O)]3+ and trans-[Pd(H2O)2(H3O)Cl]2+ complexes. The replacement of the H3O+ ion by the Cl− ion at the first three steps is thermodynamically more advantageous than the displacement of water molecules from the metal coordination sphere. The logarithms of stepwise stability constants of Pd(II) chloro complexes are found to correlate linearly with the enthalpies (ZINDO/1, PM3) of reactions of H2O replacement by Cl−.