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

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)(5)NO2](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 +/- A 0.1 A degrees C. Spectroscopic evidence is presented for the protonated oxidant. Protonation constants of 360.43 and 563.82 dm(3) 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 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: a dagger H (not equal) (kJ K-1 mol(-1)) = +113.4 +/- A 0.4 and +119 +/- A 0.3; a dagger S (not equal) (J K-1) = +107.6 +/- A 1.3 and 125.0 +/- A 1.6; a dagger G (not equal) (kJ K-1) = +81 +/- A 0.4 and +82.4 +/- A 0.4; and E (a) (kJ mol(-1)) = 115.9 +/- A 0.5 and 122.3 +/- A 0.6 for LL = bipy and phen, respectively. Effect of added anions (Cl-, and ) on the systems showed decrease in the electron transfer rate constant. An outer-sphere mechanism is proposed for the reaction.