Solvent effects on the relative stability of the PdCl_2(H_2O)_n and PdHCl(H_2O)_n cis and trans isomers

摘要

The relative stability of the cis and trans isomers of the PdCl_2(H_2O)_n and PdHCl(H_2O)_n complexes with up to six water ligands has been studied. Optimized geometries are discussed with particular emphasis on the positions taken by the second sphere water ligands. It is found that the empty coordination site above palladium is not as attractive for the second sphere ligands as might have been anticipated. Two types of hydrogen bonds, of O-H...Pd and O-H...H-Pd were found, which have only recently been detected experimentally. The effect of a water solvent on the complexes was then studied using a model with a spherical cavity surrounded by a dielectric continuum. Since the cis forms of the complexes have large dipole moments, the dipole term of the solvent reaction field strongly favours these structures. However, it is found that the quadrupolar terms have an almost equally large effect in the opposite direction, favouring the trans isomer. Even the octapole and hexadecapole moments make non-negligible contributions to the relative stabilities.