A quantum chemical consideration of ligand exchange in palladium(ii) aqueous and chloride complexes

Abstract

The behavior of potassium tetrachloropalladate(II) in media simulating biological fluids has been studied. In aqueous solutions of NaCl, the aquation rate is higher than the rate of chloro ligand introduction into the internal coordination sphere of palladium. In HCl solutions, on the contrary, the process of palladium chloro complex formation predominates. The latter is apparently due to protonation of water molecules composing aqua complexes. By means of the ZINDO/1 method, the substitution of ligands – water molecules and hydronium ion – in planar complexes of palladium(II) by chloride ion has been investigated. All complexes containing H₂O and H₃O⁺ ligands, other than [Pd(H₂O)₄]²⁺, have intramolecular hydrogen bonds. In [Pd(H₂O)₃(H₃O)]³⁺ and trans-[Pd(H₂O)₂(H₃O)Cl]²⁺, a “non-classic” symmetric hydrogen bond O ··· H ··· O is established (ZINDO/1, RHF/STO-6G*). By the first three steps the substitution of hydronium ion in the internal sphere of palladium atom is more favorable thermodynamically, compared to water molecules. Logarithms of stepwise stability constants of palladium(II) chloride complexes correlate linearly to enthalpies (ZINDO/1, PM3) of water substitution by chloride ion.

Keywords:

• Palladium(II) complexes
• Ligand exchange
• Quantum chemical consideration
• Theory–experiment correlation

Acknowledgments

The authors would like to thank Dr. Asya D. Shebaldova, Senior Researcher of Scientific-Research Institute of Chemistry Attached to the N. G. Chernyshevskii Saratov State University, for valuable advice and discussion, and Dr. Olga M. Tsivileva, Senior Researcher of the Institute of Biochemistry of Plants and Microorganisms, Russian Academy of Sciences, Saratov, Russia, for technical assistance.