Kinetics and Mechanism of Electron Transfer Reaction of an Adipato Bridged iron(III)-salen Complex with Dithionite ion in Perchloric Acid Medium

Pius O. Ukoha, Pure and Industrial Chemsitry, University of Nigeria, Nsukka, Nsukka, Nigeria
Simeon Atiga, Pure and Industrial Chemsitry, University of Nigeria, Nsukka, Nsukka, Nigeria
Oguejiofo T. Ujam, Pure and Industrial Chemsitry, University of Nigeria, Nsukka, Nsukka, Nigeria

Redox kinetics of the reaction of an adipato bridged iron(III)-salen complex, [(Fe(salen))₂adi] with dithionite ion , S₂O₄^2-, was investigated in perchloric acid at I = 0.05 mol dm^-3 (NaClO₄) and T = 29 ± 1 °C. Spectrophotometric titrations indicated consumption of one mole of S₂O₄^2- per a mole of [(Fe(salen))₂adi] reduced. Under pseudo-first order conditions of [S₂O₄^2-] above ten-fold excess of concentration of [(Fe(salen))₂adi], observed rates increased with increase in [S₂O₄^2-] and second order rate constants were fairly constant (0.285 ± 0.01 dm³ mol^-1 s^-1) indicating first order dependence of the rate on [(Fe(salen))₂adi]. A plot of logk_obs versus log[S₂O₄^2-] was linear and gave a slope of 1.0 indicating first order dependence of the rate on [S₂O₄^2-]. The reaction rate increased with increase in [H⁺] within 3 x 10^-3 mol dm^-3 ≤ [H⁺] ≤ 14 x 10^-3 mol dm^-3. The reaction was unaffected by variation of ionic strength and dielectric constant of the medium. Addition of anion and cation did not catalyze the reaction. The reaction has been analyzed on the basis of an inner-sphere mechanism mediated by proton transfer.