Kinetic Studies of the Reactions of the Ferricyanide Ion with Cobalt(II)-Aminopolycarboxylate Complexes

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Kinetic studies of the reactions of the ferricyanide ion with cobalt(II)-aminopolycarboxylate complexes were carried out. The aminopolycarboxylates used in this study were trimethylenediaminetetraacetate (TRDTA), trans-1,2-cyclohexanediaminetetraacetate (CyDTA), propylenediaminetetraacetate(PDTA), and N-hydroxyethyl ethylenediaminetriacetate(HEDTA). The first step in the reaction is the rapid formation of a binuclear complex intermediate, [(ammopolycarboxylate)CoIIINCFeII(CN)5]n−(n=5 for TRDTA, CyDTA and PDTA and 4 for HEDTA), followed by the dissociation of the intermediate to ferrocyanide and Co(III)-aminopolycarboxylate ions. The equilibrium constant of the formation reaction and the rate constant of the dissociation reaction of the intermediate at an ionic strength of 0.66 were 6.2×102 M−1 and 2.0×10−2 s−1 respectively for the TRDTA system at 5 °C, 3.3×10 M−1 and 2.7×10−2 s−1 for the CyDTA system at 25 °C, and 2.4×103 M−1 and 3.0×10−3 s−1 for the PDTA system at 25 °C. The rate constant of the dissociation of [hedta CoIIINCFeII(CN)5]4− was found to be smaller than 1×10−4 s−1 at 25 °C and at an ionic strength of 0.66.
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