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

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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 (λ 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−]<5×10−2 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 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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Authors and Affiliations

  1. Department of Chemistry, Indian Institute of Technology, 208 016, Kanpur, U.P., India

    Radhey M. Naik & Prem C. Nigam

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  1. Radhey M. Naik
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  2. Prem C. Nigam
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Naik, R.M., Nigam, P.C. Kinetic and mechanism of pentacyanohydroxoferrate(III) formation from the reaction of [Fe2HPDTA(OH)2] with cyanide ions. Transition Met Chem 12, 261–264 (1987). https://doi.org/10.1007/BF01023544

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  • DOI: https://doi.org/10.1007/BF01023544

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