Kinetic and mechanism of pentacyanohydroxoferrate(III) formation from the reaction of [Fe2HPDTA(OH)2] with cyanide ions |
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Authors: | Radhey M. Naik Prem C. Nigam |
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Affiliation: | (1) Department of Chemistry, Indian Institute of Technology, 208 016 Kanpur, U.P., India |
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Abstract: | 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 HPDTA4– 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–]<5×10–2m. 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 HPDTA4– is first-order in [Fe(CN)5OH]3– and HPDTA4–, 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. |
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