Mechanisms of interchange reactions of halogenoplatinate(IV) complexes: the trans-dihalogenobisoxalatoplatinate(IV)–iodide reactions

Abstract

The kinetics of replacement of X^– in trans-Pt ox₂X₂^2–(ox = C₂O₄, X = Cl or Br) by iodide have been studied, and the equilibrium constant for the reduction of the di-iodo-complex by iodide has been measured. The reactions are first order with respect to both complex and iodide ions, and they are considered to go by a rate-determining reduction to Pt ox₂^2–, X^–, and XI, followed by a rapid re-oxidation to form the di-iodo-complex: k_25°(X = Cl, µ= 0·05M)= 16·8 × 10^–2 l. mole^–1 sec.^–1; k₂²_5°(X = Br, µ= 0·025M)= 5·13 × 10³ l. mole^–1 sec.^–1; ΔH^‡(X = Cl)= 2·33 ± 0·17 kcal. mole^–1; ΔH^‡(X = Br)= 4·79 ± 0·31 kcal. mole^–1; ΔS^‡(X = Cl)=–54·3 ± 0·6 cal. mole^–1 deg.^–1; ΔS^‡(X = Br)=–25·5 ± 1·2 cal. mole^–1 deg.^–1.

The reaction of iodide with the bromo-complex fits into a pattern, established by reactions of iodine with related halogeno-platinate(IV) complexes, in which the rates of reduction appear to be paralleled by the thermodynamic ease of reduction. However, all of the rate parameters for the reaction with the chloro-complex are exceptional