Kinetic studies on the reaction of Cr2+ with the µ-amido-µ-sulphato- and µ-amido-µ-selenato-bis[tetra-amminecobalt(III)] complexes

Abstract

The kinetics of the reduction (i) of the µ-amido-µ-sulphato-bis[tetra-amminecobalt(III)] complex with chromium-(II) have been studied in perchlorate media, µ= 2·0M(LiClO₄). The reaction proceeds in two stages, each involv- 2Cr²⁺+(Co^III)₂→ 2Co²⁺+(Cr^III)₂(i) ing the attack of a chromium(II) and reduction of a cobalt(III). Formation of a cobalt(III)–chromium(III) intermediate in the first stage has been studied by the stopped-flow method. The rate law is –d ln [(Co^III)₂]/dt=·k₁[Cr²⁺], and at 25 °C k₁= 7·4 /mol^–1 s^–1, ΔH₁^‡= 8·7 ± 0·1 kcal mol^–1, and ΔS₁^‡=–25·3 ± 0·3 cal mol^–1 K^–1 The second stage has been studied by conventional techniques; the rate law is –d ln [Co^III. Cr^III]/dt=(k₂+k₃[H⁺]^–1)[Cr²⁺]. At 25 °C k₂= 0·256 | mol^–1 s^–1, ΔH₂^‡= 14·6 ± 0·7 kcal mol^–1 and ΔS₂^‡=–12·2 ± 2·5 cal mol^–1 K^–1 and k₃= 0·192 s^–1, ΔH₃^‡= 13·9 ± 1·0 kcal mol^–1, and ΔS₃^‡=–15·3 ± 3·4 cal mol^–1 K^–1. Spectra of the intermediate and the final product, believed to be chromium(III) dimer, have been obtained. The first stage of the reaction of the µ-amido-µ-selenato-complex is ca. 50 times faster than that of the sulphato-complex k₁= 372 | mol^–1 s^–1, ΔH₁^‡= 7·2 ± 0·2 kcal mol^–1 and ΔS₁^‡=–22·6 ± 0·6 cal mol^–1 K^–1. Subsequent reactions could not be studied because of precipitation of selenium.