Racemization Reactions of Cobalt(III) Complexes of Ethylenediaminetetraacetate and Trimethylenediaminetetraacetate

Abstract

Abstract The kinetics of racemization reactions of ethylenediaminetetraacetatocobaltate(III), CoIIIedta−, and trimethylenediaminetetraacetatocobaltate(III), CoIIItrdta−, were investigated in acid solutions. The rate law for the racemization of (+)546CoIIIedta− is: \fracd[(+)546CoIIIedta−]dt=−2\frackr1Ka+kr2[H+]Ka+[H+][(+)546CoIIIedta−] where Ka is the equilibrium constant of the reaction: (+)546CoIII(H2O)Hedta\ightleftarrows(+)546CoIIIedta−+H+ The values of kr1 and kr2 at 120°C and at an ionic strength of 0.53 are 2.0×10−5 sec−1 and 5.6×10−4 sec−1 respectively. The rate law for the racemization of (+)589CoIIItrdta− is: \fracd[(+)589CoIIItrdta−]dt=−2k′r2[H+][(+)589CoIIItrdta−] The value of k′r2 at 120°C and at an ionic strength of 0.53 is 1.9×10−5 l mol−1 sec−1. It is known that the rate of the racemization reaction of optically-active CoIIedta2− is accelerated by the presence of CoIIedta2− because of the electron-exchange reaction between these two species. In this paper, the electron-exchange reaction between CoIIItrdta− and CoIItrdta2− is also investigated by the utilization of the optical activity. This reaction proceeds faster than the reaction between CoIIIedta− and CoIIedta2−.