Abstract
Cis-Pt(II) complexes, namely [Pt{2-(phenylthiomethyl)pyridine}(H2O)2](CF3SO3)2 Pt(pyS Ph ), [Pt{2-(4-tert-butylphenylthiomethyl)pyridine}(H2O)2](CF3SO3)2 Pt(pyS Ph( t -But) ) and [Pt{2-(4-fluorophenylthiomethyl)pyridine}(H2O)2](CF3SO3)2 Pt(pyS PhF ), were synthesised and characterised. The pK a1 and pK a2 values of the complexes were determined titrimetrically. Substitution of the aqua ligands from these complexes by thiourea nucleophiles was studied at a pH of 2 and ionic strength of 0.1 M under pseudo-first-order conditions using stopped-flow and UV–visible spectrophotometric techniques. Substitution of the aqua ligands depends on both the nature and concentration of the incoming ligand, with low enthalpy and negative entropy of activation values. Substitution of the first and second aqua ligands occurs sequentially and fits the rate laws: k obs (1/2) = k (1/2) [Nu]. The second-order rate constant, k 1, relates to the substitution trans to sulphur, while k 2 is the second-order rate constant for the subsequent substitution of the aqua ligand trans to pyridine. The rate of substitution of the first aqua ligand decreases in the order: Pt(pyS Ph( t -But) ) > Pt(pyS PhF ) > Pt(pyS Ph ), while that of the second decreases in the order: Pt(pyS Ph( t -But) ) > Pt(pyS Ph ) > Pt(pyS PhF ), reflecting the influence of the substituents on the spectator ligands. 195Pt NMR spectra of aged solutions of complexes with the thiourea nucleophile suggest a subsequent but rapid concentration-independent ring opening of the N,S-bidentate ligand to form a PtS 4 species. The crystal structure of Pt(pyS PhF )Cl 2 was elucidated by X-ray diffraction analysis.
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