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: CrII(H2O)62++ IrIVCl62– [graphic omitted] CrIII(H2O)63++ IrIIICl63–. An inner-sphere mechanism is also effective and a binuclear intermediate [(H2O)5CrClIrCl5](peaks at 354 nm., Iµ 560; 400 nm., Iµ 400; and ca. 425, Iµ∼380) has been detected. This subsequently breaks down to give mononuclear products, Cr(H2O)5Cl2+ and IrCl5H2O2–: CrII(H2O)62++ IrIVCl62– [graphic omitted] [(H2O)5CrIIIClIrIIICl5] [graphic omitted] CrIII(H2O)5Cl2++ IrIIICl5H2O2 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.