Abstract
The rate of isotopic exchange of cobalt (II) ion with cobalt (II)-EDTA complex is measurable (reaction half time of approx. 20–100 min) in a pH range of 4·8 – 6·1 and concentrations of Co(II) ion and complex of the order of 10 −3–10 −4 M. Under these conditions the exchange rate was first order in the complex concentration and varied in a complicated manner, being of partial order in the concentration of cobalt (II) and hydrogen ion. The experimental rates of exchange are expressed within experimental precision by: ▪ where k 1 = 0·070 1/mole per sec; k 2 = 11·61/mole per sec; and k 3 = 1·00 × 10 41 2/mole 2per sec; at a temperature of 20°C. Arrhenius activation energies corresponding to k 1, k 2, and k 3 evaluated from measurements at 20, 30 and 40°C were found to be, 16·0, 12·1 and 10·6 kcal/mole, respectively. The entropy of activation corresponding to k 1 was −12 eu. The substitution reaction mechanism suggested by the results appeared to involve the unprotonated chelate, Co(II)EDTA 2− as well as the species Co(II)HEDTA −. The observation that the cobalt (II)-EDTA complexes are not as labile as other transition element chelates is qualitatively in agreement with the predictions of crystal field calculations made on the assumption of a S E1 dissociative mechanism involving octahedral complexes and weak field metal-EDTA interaction.
Published Version
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