Effect of Coordinated Ligands on the Kinetics of Substitution Reactions in Metal Complexes

Abstract

In recent years considerable progress has been made in the study of the fundamental kinetic steps which occur in coordination substitution reactions. Many of the rate constants for the substitution of coordinated water in aquo - metal complexes now have been determined (1). For the general case of metal ions which are not hydrolyzed, the rate of water substitution is the rate limiting reaction step. This rate step is characteristic for each metal ion and is found in substitution reactions where the aquo-metal ion is one reactant. The next piece of fundamental information needed is the effect which other coordinated ligands have on subsequent rates of water substitution. Many substitution reactions of interest involve the replacement of water not from the aquo -metal ion but rather from a metal complex which has several coordination sites occupied by other ligands. Information is sparse on this subject for the divalent metal ions but it has been suggested that an increase in the substitution rate accompanies a decrease in the charge of the complex. Thus, the mono-glycine complexes of nickel and cobalt are more labile to the substitution of a second glycine than are the aquo-metal ions (2). The same is known also for some metal hydroxo species as compared to the corresponding aquo-metal ions, cf. (1). On the other hand, the neutral imidazole complexes of the same metals do not appear to change the rate of the water substitution steps (2).