Kinetics and mechanism of the reduction of dodecatungstocobaltate(III) by iminodiacetate, nitrilotriacetate, and ethylenediaminetetraacetate. A comparative study of the reactivity of different amine-N-carboxylates.

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The kinetics of reduction of dodecatungstocobaltate(III) by iminodiacetate (IDA), nitrilotriacetate (NTA), and ethylenediaminetetraacetate (EDTA) have been studied in aqueous solution at 50 °C, 30 °C, and 15 °C respectively. In general, the reactivity of the amine-N-carboxylates is in the order EDTA>NTA>IDA. All the anionic species have been found to undergo oxidation via both a spontaneous and an alkali metal ion-catalyzed pathway, while molecular form of the acids are oxidized only via spontaneous pathway. The rate of oxidation is given by a general expression kox=k°+k[M+]n where kox=kobs/2[L]T([L]T=total concentration of a particular amine-N-carboxylate) and n may have values 1 or 2 depending on the nature of M+. The terms k° and k account for the rate constants of the spontaneous and catalyzed paths respectively. A general trend K+>Na+>Li+ for the catalyzed path has been observed for the oxidation of these amine-N-carboxylates. Reactivities of different species present in the experimental pH range have been evaluated by carrying out experiment at different pH and using these data in appropriate rate expressions. The reactivities (50 °C) of Hida− are k°=4.70×10−5 dm3 mol−1 s−1 and k1=3.44×10−4 dm6 mol−2 s−1 for the spontaneous and Na+ catalyzed paths respectively. For NTA oxidation (30 °C), rates are k10°(H3nta)=1.11×10−3 dm3 mol−1 s−1, k11°(H2nta−, spontaneous)=7.00×10−3 dm3 mol−1 s−1, k11(H2nta−, Na+ catalyzed)=3.50×10−2 dm6 mol−2 s−1, k12°(Hnta2−, spontaneous)=7.20×10−3 dm3 mol−1 s−1, k12(Hnta2−, Na+ catalyzed)=4.01×10−2 dm6 mol−2 s−1. The corresponding reactivities (15 °C) for EDTA oxidation are k20°(H4edta, spontaneous)=6.80×10−3 dm3 mol−1 s−1, k21°(H3edta−, spontaneous)=4.80×10−2 dm3 mol−1 s−1, k21(H3edta−, Na+ catalyzed)=0.48 dm9 mol−3 s−1, k22°(H2edta2−, spontaneous)=0.21 dm3 mol−1 s−1, k22(H2edta2−, Na+ catalyzed)=1.70 dm6 mol−2 s−1, k23°(Hedta3−, spontaneous)=1.71 dm3 mol−1 s−1, k23(Hedta3−, Na+ catalyzed)=30.00 dm6 mol−2 s−1. A plausible mechanism considering an outersphere association between the complex and reductant has been suggested where the alkali metal ions are assumed to act as a bridge between the reactants.
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K. Saha
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science

Kinetics and mechanism of the reduction of dodecatungstocobaltate(III) by iminodiacetate, nitrilotriacetate, and ethylenediaminetetraacetate. A comparative study of the reactivity of different amine-N-carboxylates.

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