Electron-Transfer Reactions in Non-Aqueous Media. V. Solvent Effects in the Reduction of CoN₃(NH₃)₅²⁺ by Iron(II)

Abstract

The reduction of CoN₃(NH₃)₅²⁺ by iron(II) is rate-determined by a two-stage process involving the reversible formation of an azide-bridged precursor complex prior to electron transfer in each of the solvents water, Me₂SO, aqueous Me₂SO and HCONMe₂. The activation parameters in H₂O and Me₂SO, and the trends shown with increasing Me₂SO concentrations in aqueous Me₂SO, are similar to the properties of the previously studied CoCl(NH₃)₅²⁺ and CoBr(NH₃)5²⁺ systems and contrast with the reduction of COF(NH₃)₅²⁺. The results are consistent with a bridged precursor complex octahedral at both the iron and cobalt atoms in water but with tetrahedral coordination about the iron in Me₂SO. In HCONMe₂, as in the reduction of COF(NH₃)₅²⁺, COCl(NH₃)₅²⁺ and COBr(NH₃)₅²⁺, the precursor complex is a significant part of the reacting solutions, and as a result the experimental pseudo-first-order rate constants for the loss of Co^III are not linearly dependent on the concentration of Fe^II. The initial spectra of the reacting solutions in this system also indicate significant concentrations of the precursor complex.