Intermediate and ion-pair formation in the outer-sphere reactions between azido-pentacyanocobaltate(III) and iron(II) polypyridyl complexes in aqueous medium

The kinetics of the reactions between azido-pentacyanocobaltate(III), Co(CN) 5 N 3 3− , and iron(II) polypyridyl complexes, Fe(LL) 3 2+ (LL = bipy, phen), have been studied in both neutral and acidic aqueous solutions at I = 0.1 mol dm −3 NaCl. The reactions were carried out under pseudo-first-order conditions in which the concentration of Fe(LL) 3 2+ was kept constant, and the second-order rate constants obtained for the reactions at 35 °C were within the range of 0.156–0.219 dm 3 mol −1 s −1 for LL = bipy and 0.090–0.118 dm 3 mol −1 s −1 for LL = phen. Activation parameters were measured for these systems. The dependence of reaction rates on acid was studied in the range [H + ] = 0.001–0.008 mol dm −3 . The reaction in acid medium shows interesting kinetics. Two reactive species were identified in acid medium, namely, the protonated cobalt complex and the azido-bridged binuclear complex. The electron-transfer process is proposed to go by mixed outer- and inner-sphere mechanisms in acid medium, in which electron transfer through the bridged inner-sphere complex ( k 5 ) is slower than through the outer-sphere path ( k 4 ).