Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from mono(aminocarboxylato)iron(III) complexes

Abstract

The kinetics and mechanism of the system: [FeL(OH)]2−n + 5 CN− ⇌ [Fe(CN)5(OH)]3− + Ln−, where L=DTPA or HEDTA, have been investigated at pH= 10.5±0.2, I=0.25 M and t=25±0.1 ‡C. As in the reaction of [FeEDTA(OH)]2−, the formation of [Fe(CN)5(OH)]3− through the formation of mixed ligand complex intermediates of the type [FeL(OH)(CN)x]2−n−x, is proposed. The reactions were found to consist of three observable stages. The first involves the formation of [Fe(CN)5(OH)]3−, the second is the conversion of [Fe(CN)5(OH)]3− into [Fe(CN)6]3− and the third is the reduction of [Fe(CN)6]3− to [Fe(CN)6]4− by oxidation of Ln− The first reaction exhibits a variable order dependence on the concentration of cyanide, ranging from one at high cyanide concentration to three at low concentration. The transition between [FeL(OH)]2−n and [Fe(CN)5(OH)]3− is kinetically controlled by the presence of four cyanide ions around the central iron atom in the rate determining step. The second reaction shows first order dependence on the concentration of [Fe(CN)5(OH)]3− as well as on cyanide, while the third reaction follows overall second order kinetics; first order each in [Fe(CN)6]3− and Ln−, released in the reaction. The reaction rate is highly dependent on hydroxide ion concentration. The reverse reaction between [Fe(CN)5(OH)]3− and Ln− showed an inverse first order dependence on cyanide concentration along with first order dependence each on [Fe(CN)5− (OH)]3− and Ln−. A five step mechanism is proposed for the first stage of the above two systems.