Kinetics and mechanism of the oxidation of tris(2,2′-bipyridine)iron(II) and tris(1,10-phenanthroline)iron(II) complexes by nitropentacyanocobaltate(III) in acidic aqueous medium

ABSTRACT The kinetics of the oxidation of tris(2,2′-bipyridyl)iron(II) and tris(1,10-phenanthroline)iron(II) complexes ([Fe(LL)3]2+, LL = bipy, phen) by nitropentacyanocobaltate(III) complex [Co(CN)5NO2]3− was investigated in acidic aqueous solutions at ionic strength of I = 0.1 mol dm−3 (HCl/NaCl). The reactions were carried out at fixed acid concentration ([H+] = 0.01 mol dm−3) and the temperature maintained at 35.0 ± 0.1 °C. Spectroscopic evidence is presented for the protonated oxidant. Protonation constants of 360.43 and 563.82 dm3 mol−1 were obtained for the monoprotonated and diprotonated Co(III) complexes respectively. Electron transfer rates were generally faster for [Fe(bipy)3]2+ than [Fe(phen)3]2+. The redox complexes formed ion-pairs with the oxidant with increasing concentration of the oxidant over that of the reductant. Ion-pair constants for these reaction were 160.31 and 131.9 dm3 mol−1 for [Fe(bipy)3]2+ and [Fe(phen)3]2+, respectively. The activation parameters measured for these systems have values as follows: ∆H ≠ (kJ K−1 mol−1) = +113.4 ± 0.4 and +119 ± 0.3; ∆S ≠ (J K−1) = +107.6 ± 1.3 and 125.0 ± 1.6; ∆G ≠ (kJ K−1) = +81 ± 0.4 and +82.4 ± 0.4; and E a (kJ mol−1) = 115.9 ± 0.5 and 122.3 ± 0.6 for LL = bipy and phen, respectively. Effect of added anions (Cl−, $ {\text{SO}}_{4}^{2 - } $ and $ {\text{ClO}}_{4}^{ - } $ ) on the systems showed decrease in the electron transfer rate constant. An outer-sphere mechanism is proposed for the reaction.