An Insight into the Kinetics and Mechanism of Oxidation of Tris(4,4'-dimethyl-2,2'-bipyridine)iron(II) by Bromate

Kinetic investigation of oxidation of tris(4,4'-dimethyl, 2,2'-bipyridine)iron(II) by bromate has been undertaken. The redox reaction between [Fe(dmbpy)3]2+ and bromate ion (BrO$$_{3}^{ - }$$) was monitored spectrophotometrically under the pseudo-first order condition i.e.[BrO$$_{3}^{ - }$$] ≫ [Fe(dmbpy)3]2+. Kinetic data revealed that the pseudo first order rate constant (kobs) is independent of the concentration of [Fe(dmbpy)3]2+. Though, when concentration of bromate ion is increased in reaction mixture at fixed pH, the rate also increases up to the saturation point at higher concentrations, indicating a precursor complex formation and an outer-sphere mechanism. The influence of accelerating [H+] and ionic strength on reaction rate were also investigated. The increasing values of the rate constant at low pH mention the involvement of protonated bromate species (HBrO3 and H2BrO$$_{3}^{ + }$$) in the rate-determining step. However, a rise in the value of the rate constant with increasing ionic strength implies the diprotonated species, H2BrO$$_{3}^{ + }$$, as the reactive species. On the basis of these conclusions, we were able to postulate the appropriate mechanism and the rate law of this redox reaction.