Kinetic and mechanism of pentacyanohydroxoferrate(III) formation from the reaction of [Fe2HPDTA(OH)2] with cyanide ions

Summary The kinetics and mechanism of exchange of HPDTA in Fe₂HPDTA(OH)₂] with cyanide ion (HPDTA=2-hydroxytrimethylenediaminetetraacetic acid) was investigated spectrophotometrically by monitoring the peak at 395 nm ( _max of Fe(CN)₅OH]^3– at pH=11.0±0.02,I=0.25m (NaClO₄) at ±0.1°C).Three distinct observable stages were identified; the first is the formation of Fe(CN)₅OH]^3–, the second the formation of Fe(CN)₆]^3– from it and the third the reduction of Fe(CN)₆]^3– to Fe(CN)₆]^4– by HPDTA^4– released in the first stage.The first stage follows first-order kinetics in Fe₂HPDTA(OH)₂] and second-order in CN^–] over a wide range of CN^–], but becomes zero order at CN^–]<5×10^–2 m. We suggest a cyanide-independent dissociation of Fe₂HPDTA)(OH)₂] into FeHPDTA(OH)] and Fe(OH)]²⁺ at low cyanide concentrations and a cyanide-assisted rapid dissociation of Fe₂HPDTA(OH)₂] to FeHPDTA(OH)(CN)]^3– and Fe(OH)]²⁺ at higher cyanide concentrations. The excess of cyanide reacts further with FeHPDTA(OH)(CN)]^3– finally to form Fe(CN)₅OH]^3–.The reverse reaction between Fe(CN)₅OH]^3– and HPDTA^4– is first-order in Fe(CN)₅OH]^3– and HPDTA^4–, and exhibits inverse first-order dependence on cyanide concentration.A six-step mechanism is proposed for the first stage of reaction, with the fifth step as rate determining.