Identification of inner- and outer-sphere paths in the reaction of chromium(II) with hexachloroiridate(IV), and the kinetics of the decomposition of the binuclear intermediate

Abstract

The one-equivalent reaction of chromium(II) with hexachloroiridate(IV) has been studied at temperatures in the range 0–25°, µ= 1·0M(LiClO₄). Part of the reaction is accounted for by an outer-sphere path: Cr^II(H₂O)₆²⁺+ Ir^IVCl₆^2– [graphic omitted] Cr^III(H₂O)₆³⁺+ Ir^IIICl₆^3–. An inner-sphere mechanism is also effective and a binuclear intermediate [(H₂O)₅CrClIrCl₅](peaks at 354 nm., ε 560; 400 nm., ε 400; and ca. 425, ε∼380) has been detected. This subsequently breaks down to give mononuclear products, Cr(H₂O)₅Cl²⁺ and IrCl₅H₂O^2–: Cr^II(H₂O)₆²⁺+ Ir^IVCl₆^2– [graphic omitted] [(H₂O)₅Cr^IIIClIr^IIICl₅] [graphic omitted] Cr^III(H₂O)₅Cl²⁺+ Ir^IIICl₅H₂O² The rate of decomposition of the intermediate and the ratio of products [Cr(H₂O)₆³⁺]:[Cr(H₂O)₅Cl²⁺] are independent of hydrogen-ion concentration over the range [H⁺]= 0·11 to 1·0M. At 25°k₃= 4·23 × 10^–2 sec.^–1, ΔH₃‡= 21·8 (±1·0) kcal. mole^–1 and ΔS₃‡= 8·4 (±3·0) e.u., and the rate constants k₁ and k₂ are > 10⁶ l. mole^–1 sec.^–1. From the temperature dependence of the product ratio [Cr(H₂O)₆²⁺]:[Cr(H₂O)₅Cl²⁺] the differences in activation parameters for the inner and outer-sphere paths are ΔH₂‡–ΔH₁‡= 5·7 (±0·7) kcal. mole^–1 and ΔS₂‡–ΔS₁‡= 19·0 (±2·0) e.u. Possible implications of these values are considered.