Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from [FeL(OH)2]? complex, (L=N-(2-hydroxyethyl) iminodiacetate anion

Abstract

The kinetics and mechanism of the system [FeHIDA-(OH)2]−+5CN−⇌[Fe(CN)5OH−+HIDA2−+OH− (HIDA=N-(2-hydroxyethyl) (iminodiacetate) at pH=9.5±0.02, I=0.1 M and at 25±0.1°C have been studied spectrophotometrically at 395 nm (λmax of [Fe(CN)5OH]3−]. The reaction has three distinguishable stages; the first is formation of [Fe(CN)5OH]3−, the second is conversion of [Fe(CN)5OH]3− into [Fe(CN)6]3−, and last is the reduction of [Fe(CN)6]3− to [Fe(CN)6]4− by the HIDA2− released in the first stage. The first stage shows variable-order dependence on cyanide concentration, unity at high cyanide concentration and zero at low cyanide concentration. The second stage exhibits first-order dependence on the concentration of [Fe(CN)5OH]3− as well as on cyanide. The reverse reaction between [Fe(CN)5OH]3− and HIDA2− is first-order in each of these species and inverse first-order in cyanide. On the basis of forward and reverse rate studies, a five-step mechanism has been proposed for the first stage. The first step involves a slow loss of one OH−, by a cyanide-independent path.