Effects of anions on redox reactions. 2. Chromium(II) reduction of [Co(sep)]³⁺ (sep = sepulchrate) in the presence of halide ions

Temperature-dependence studies on the Cr(II) reduction of [Co(sep)]³⁺ (I = 0.5 M (LiClO₄)) provided the activation parameters ΔH^‡ = 9 ± 2 kcal mol^-1 and ΔS^‡ = -43 ± 5 eu. In a chloride medium (I = 0.5 M (LiCl + LiClO₄)) the reaction was markedly accelerated, with ΔH^‡ = 11 ± 2 kcal mol^-1 and ΔS^‡ = -28 ± 5 eu. The reaction was also carried out in the presence of bromide and iodide ions, showing significant rate enhancements. Analysis of experimental parameters suggests that the anion catalysis is best explained in terms of reduction of the entropic contribution to the activation energy through hydrogen-bonding interactions, originating from outer-sphere complexation of the reactants with the anions. The catalyzed rate constants for electron transfer seem to be the same for all the halides, with a value of (2.5 ± 0.3) × 10^-2 M^-1 s^-1, whereas the ion-pairing formation constants at I = 0.5 M were estimated to be 5.5, 2.3, and 1.7 M^-1 for Cl^-, Br^-, and I^-, respectively.