Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from the reaction of cyanide ion with [Fe2L(OH)2]2? (L=triethylenetetraaminehexaacetate)

Summary The kinetics and mechanism of the reaction between Fe₂L(OH)₂]^2– and cyanide ion (L = TTHA, triethylenetetraaminehexaacetate) have been studied spectrophotometrically atpH=11.0±0.1,I=0.1 M(NaClO₄) and T = 25±0.1 °C. The overall reaction consists of three distinct, observable stages. The first stage involves the dissociation of the binuclear complex into a mononuclear complex FeL(OH)]^4– which then reacts with cyanide to form Fe(CN)₅OH]^3–. The species Fe(CN)₅OH]^3– reacts further with an excess of cyanide and forms Fe(CN)₆]^3– in the second stage of reaction. The last stage involves the reduction of Fe(CN)₆]^3– formed in the second stage by the TTHA^6– released in the first stage of reaction. The formation of Fe(CN)₅OH]^3– in the first stage is firstorder in Fe₂L(OH)₂]^2– and third-order in cyanide over a large range of cyanide concentrations but becomes zero-order in cyanide at CN^–] < 4×10^–2M.These observations enable us to suggest the presence of a slow step in which Fe₂L(OH)₂]^2– dissociates into FeL(OH)]^4– and FeOH]²⁺ at low cyanide concentrations and a cyanide assisted rapid dissociation of Fe₂L(OH)₂]^2– to FeL(OH)(CN)]^5– at higher cyanide concentrations. The species FeL(OH)(CN)]^5– reacts further with an excess of cyanide to produce Fe(CN)₅OH]^3– finally.The reverse reaction between Fe(CN)₅OH]^3– and TTHA^6– follows first-order dependence in each of Fe(CN)₅OH]^3– and TTHA^6– and inverse first-order dependence on cyanide concentration. A six-step mechanism has been proposed for the first stage of reaction in which the fifth has been identified as the rate-determining step.