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++(CoIII)2→ 2Co2++(CrIII)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 [(CoIII)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 [CoIII. CrIII]/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.