The Solvent Path in Square-Planar Substitutions. Kinetics and Mechanism for Reactions of Tetrachloroplatinate(II) and Aquachloroplatinates(II) with Halides, Thiocyanate and Dimethyl Sulfoxide

The kinetics for the reactions between the six complexes PtCln(H2O)4-n−2n, n = 0,1,2,3,4 (including cis- and trans-isomers for n=2) and the entering ligands Y = Cl−, Br−, I−, SCN−, and DMSO have been studied. The experiments with PtCl42− and trans- PtCl2(H2O)2 give the usual two-term rate constant kexp = k1 + k2[Y], when no extra chloride has been added to the solutions. The parameter k1 can be identified as the acid hydrolysis rate constant for PtCl42− and trans-PtCl2(H2O)2, respectively. It is shown that both PtCl3H2O− and PtCl(H2O)3+ are sufficiently reactive for all Y to be the intermediates in the k1- paths of these two reactions. An associative process with the solvent is therefore probable. The entering ligand order for H2O as leaving ligand is DMSO < Cl− < Br− < SCN− < I− (approximately 0.3:1:4:40:100) for the substrate complexes studied. DMSO is a bad entering ligand in spite of its large trans-effect. The relative trans-effect Cl−/H2O and the relative efficacy of H2O and Cl− as leaving ligands depend on the nature of the entering ligand Y, whereas the relative cis-effect Cl-/H2O is independent of Y. This is compatible with an associative mechanism via a trigonal bipyramidal intermediate.