Mechanism of the oxidation of the molybdenum(V)–ethylenediaminetetra-acetato dimer by hexachloroiridate(IV) and tris(1,10-phenanthroline)iron(III)

Abstract

The kinetics of the oxidation of µ-NN′-ethylenediaminetetra-acetato-di-µ-oxo-bis[oxomolybdenurn(V)], [Mo₂O₄(edta)]^2–, by hexachloroiridate(IV), [IrCl₆]^2–, have been studied in aqueous perchloric acid solutions, I= 1.0M (LiClO₄), at 15–30 °C. The reaction is first-order in both reactants and is independent of [H⁺] in the range 0.01–0.50M investigated. Pseudo-first-order rate constants. k_obs, obtained by monitoring the disappearance of [IrCl₆]^2– are substantially lower in the presence of added hexachloroiridate(III), [IrCl₆]^3–. The experimental data give a good fit to (i). –d[Ir^IV]//_dt=k_obs[Ir^IV]=2k₁k₂[(Mo^V)₂][Ir^IV]//_{k–1[Ir^III]+k2(i)} where the rate constants k₁, k_–1 and k₂ are as defined in the reaction scheme (ii)–(iv). (Mo^V)₂+ Ir^IV [graphic omitted] Mo^VMo^VI+ Ir^III(ii) Mo^VMo^VI [graphic omitted] Mo^V+ Mo^VI(iii) Mo^V+ Ir^IV [graphic omitted] Mo^VI+ Ir^III(iv) At 25 °C,k₁= 6.6 l mol^–1 s^–1, ΔH^‡₁= 13.3 ± 0.3 kcal mol^–1, ΔS^‡₁=–10.0 ± 1.0 cal K^–1 mol^–1, and k_–1/k₂= 1.74 × 10³ l mol^–1. The corresponding oxidation with tris(1,10-phenanthroline)iron(III), [Fe(phen)₃]³⁺, exhibits similar features with (at 25 °C)k₁= 1.9 × 10³ l mol^–1 s^–1, ΔH^‡₁= 16.6 ± 0.4 kcal mol^–1, ΔS^‡₁= 12.2 ± 1.3 cal K^–1 mol^–1, and k_–1/k₂= 1.17 × 10⁴ l mol^–1. No reaction of [Mo₂O₄(edta)]^2– with the strong two-equivalent, oxidant hexachloroplatinate(IV), [PtCl₆]^2–, was observed during a period of 1 day at 25 °C.