Substitution kinetics of chelate complexes as studied by isotopic exchange

The fact that isotopic exchange reactions take place at chemical equilibrium makes this type of reaction useful for kinetic studies. In the case of chelate complexes [M/chel/sub(n)] isotopic exchange reactions can be classified as metal exchange reactions and ligand exchange reactions. The literature on reactions of mainly transition metal complexes is reviewed with respect to the kinetic information obtained. The value and the limitations of isotopic exchange studies as a kinetic technique are discussed. The normal technique for following isotopic exchange reactions makes necessary to separate the exchange partners after certain intervals of reaction time and to measure the change in the isotopic composition of one or both partners. The most common separation methods are separation by extraction, precipitation, or chromatographic (ion exchange) adsorption. By the application of automatic devices instead of manual sampling and extracting the time necessary for separating one of the exchange partners by extraction can be reduced to a few seconds. As a consequence, half times of exchange down to approximately four to five seconds, can be accurately determined. For faster exchange reactions, however, the common techniques for separating the exchange partners become inadequate. (T.G.)