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

Elding, Lars IvarLund University,Lunds universitet,Centrum för analys och syntes,Kemiska institutionen,Institutioner vid LTH,Lunds Tekniska Högskola,Centre for Analysis and Synthesis,Department of Chemistry,Departments at LTH,Faculty of Engineering, LTH (författare)

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.

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