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

摘要

The kinetics of the oxidation of tris(2,2'-bipyri-dyl)iron(II) and tris(l,10-phenanthroline)iron(II) complexes ([Fe(LL)3]~(2+), LL = bipy, phen) by nitropentacyanocobal-tate(m) complex [Co(CN)5NO2]~(3-) was investigated in acidic aqueous solutions at ionic strength of I = 0.1 mol dm~(-3) (HC1/ 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. Profanation constants of 360.43 and 563.82 dm~3 mol~(-1) were obtained for the monoprotonated and diprotonated Co(HI) 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 dm~3 mol~(-1) for [Fe(bipy)3]~(2+) and [Fe(phen)3]~(2+) respectively. The activation parameters measured for these systems have values as follows: ΔH~≠ (kJ K~(-1) mol~(-1)) = +113.4 ±0.4 and +119 ±0.3; ΔS~≠ (J K~(-1)) = +107.6 ± 1.3 and 125.0 ± 1.6; AG* (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 (Cr~-,SO4~(2-) and ClO4~-) on the systems showed decrease in the electron transfer rate constant. An outer-sphere mechanism is proposed for the reaction.