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 CoIII are not linearly dependent on the concentration of FeII. The initial spectra of the reacting solutions in this system also indicate significant concentrations of the precursor complex.