Kinetics of Multidentate Ligand Substitution Reactions. XVI. Substitution Reactions of l,2-Cyclohexanediamine-N,N,N′,N′-tetraacetic Acid (CyDTA) with the Zinc(II)–Ethylenediaminemonoacetate (EDMA) and -Ethylenediamine-N,N′-diacetate (EDDA) Complexes and That of Ethylenediamine-N,N,N′,N′-tetraacetic Acid (EDTA) with the Nickel(II)–EDDA Complex

The kinetics of the substitution reactions of CyDTA with zinc(II)–EDMA and –EDDA complexes and that of EDTA with the nickel(II)-EDDA complex were studied systematically by using the polarographic technique. All the substitution reactions were found to be first-order with respect to the CyDTA or EDTA anion and also first-order with respect to the 1 : 1-ratio EDMA or EDDA complex. From a comparison of the observed rate constants with those estimated on the basis of the proposed reaction intermediates, the detailed reaction mechanisms were discussed. The rates of the reactions of zinc(II)–EDMA complex and nickel(II)–EDDA complex could be understood in terms of the ethylenediamine reaction mechanism, but that of the reaction of the zinc(II)–EDDA complex with the CyDTA anion could not. This discrepancy may be ascribed by the steric hindrance of the CyDTA anion to the formation of an ethylenediamine reaction intermediate in the reaction of the zinc(II)–EDDA complex.