Ligand exchange and substitution at platinum(II) complexes: evidence for a dissociative mechanism

Square-planar complexes of the type cis-[Pt(Me)2(Me2SO)(PR3)] (1–6) where PR3 represents a series of isosteric tertiary phosphanes [P(4-MeOC6H4)3, P(4-MeC6H4)3, P(C6H5)3, P(4-FC6H4)3, P(4-ClC6H4)3, P(4-CF3C6H4)3] have been synthesised and fully characterised through elemental analysis, 1H and 31P{1H} NMR. The coupling constants 1JPtP with the isotopically abundant 195Pt (33%, I=1/2) of 1–6, as those of the pyridine cis-[Pt(Me)2(py)(PR3)] derivatives (7–12), show linear dependencies on the basicity of the coordinated phosphane. The rates of dimethyl sulfoxide exchange for all the complexes have been measured at relatively low temperatures by 1H NMR isotopic labelling experiments with deuterated chloroform as the solvent. Pyridine for dimethyl sulfoxide substitution has been studied at higher temperatures through conventional spectrophotometric techniques. The rates of both processes show no dependence on ligand concentration, for each complex the value of the rate of ligand substitution is in reasonable agreement with the value of the rate of ligand exchange at the same temperature, and the kinetics are characterised by largely positive entropies of activation. There is a compensation-effect between ΔH‡ and ΔS‡, i.e., a greater ΔH‡ is accompanied by a larger positive ΔS‡, indicating that all complexes react via the same mechanism. The basicity of the phosphane does not affect significantly the reaction rates. The general pattern of behaviour indicates that the rate determining step for substitution is the dissociation of the sulfoxide ligand and the formation of a three-coordinated [Pt(Me)2(PR3)] uncharged intermediate.