CYCLIC VOLTAMMETRIC STUDY OF RATE OF LIGAND EXCHANGE BETWEEN CADMIUM ION AND CALCIUM ETHYLENEDIAMINETETRAACETATE

The rate of ligand exchange between calcium ethylenediaminetetraacetate complex ion and cadmium ion is measured using the technique known as cyclic stationary electrode polarography. In the presence of excess calcium ion the reaction is found to proceed through (1) a pathway involving a fast, acid-catalyzed dissociation of the calcium complex followed by a rate- limiting combination of the free monoprotonated ligand with cadmium ion and (2) a pathway involving a direct attack of the complex by cadmium ion. At 25° the second-order rate constant for the first pathway is 2.6 ± 0.7 × 108 M−1 sec−1 and for the second pathway is 2.3 ± 0.2 × 102 M−1 sec−1. The direct reaction is found to proceed through an intermediate in which the multidentate ligand is half-unwrapped and partially bound to both calcium and cadmium ions. The rate- limiting step in both pathways appears to be the formation of the initial cadmium-nitrogen bond. © 1968, American Chemical Society. All rights reserved.