The chromium(II)-catalysed dissociation of monohalogenochromium(III) complexes and the effect of non-bridging ligands on electron-transfer reactions

Abstract

The reactions of chromium(II) with monobromochromium(III) and monoiodochromium(III) which result in a net dissociation of the bromide and iodide have been studied in aqueous perchloric acid solutions, ionic strength µ= 2·0M, by spectrophotometric techniques. In each case the kinetics conform to a rate equation of the type Rate =k3′[Cr2+][CrX2+][H+]–1 corrections being necessary for the chromium(II)-independent aquation of CrX2+. A rate equation of the same form has previously been reported for the reaction of CrCl2+. In each case the reactants are believed to be Cr2+ and Cr(X)OH+, and the activated complex [CrOHCrX]3+. In the reaction with CrF2+ there is in addition a hydrogen-ion-independent path which is believed to correspond to the activated complex [CrOHCrHF]4+. At 25° the non-bridging anions have a large (ca. 104) effect on rates, I–(2·13 × 10–2 sec.–1) > Br–(1·74 × 10–3 sec.–1) > Cl–(3·2 × 10–4 sec.–1) > F–(ca. 1·6 × 10–6 sec.–1). This order can be accounted for by considering the relative ligand-field strengths of the halide ions and the ease with which Cr–X bonds can be stretched, but other factors may be important. The effect is of about the same order of magnitude as that of bridging halide ions in the reactions of Cr2+ with CrX2+ and of Cr2+ with Cr(NH3)5X2+. Other data for the reactions of Cr(NH3)OH2+ and Cr(H2O)OH2+ with Cr2+ are considered.