On the reactivity of precursor complexes in the system pentacyanoferrate(II) and pentaammine(dimethylsulfoxide)cobalt(III)

Abstract

In the search for evidence of precursor complexes in electron transfer reactions, a binuclear intermediate was detected in the system pentaammine(dimethylsulfoxide)cobalt(III) and aquopentacyanoferrate(II). Cyclic voltammetry measurements and careful analysis of the products indicated that the formation of a sulfuriron bond stabilizes the intermediate with respect to electron transfer reaction. Interpretation of the kinetics data is consistent to the formation of an outer sphere complex (K = 350 M −1) which undergoes internal substitution (k = 20 s −1 to yield the dimethyl sulfoxide bridged intermediate. The binuclear intermediate aquates by breaking the cobalt(III)sulfoxide bond, with a specific rate of 0.106 s −1. ΔH + = 20 Kcal mol −1 and ΔS ≠ = 4 cal mol −1 deg −1, at 25 °C and μ = 0.10 M (lithium perchlorate). Using the less labile amminopentacyanoferrate(II) ion instead of the aquopentacyanoferrate(II) complex, an outer sphere electron transfer reaction with saturation behavior has been observed. For this system, an ion pair association constant of 470 M −1 (ΔH = +2 Kcal mol −1 and ΔS = 20 cal mol −1 deg −1) and an intramolecular transfer rate of 1.22 s − (ΔH ≠ = 22 Kcal mol −, ΔS ≠ = 17 cal mol −1 deg −1) have been obtained at 25 °C, μ = 0.10 M (lithium perchlorate).