Substitution Processes in Simple and Complicated Metal Complexes

The mechanisms of substitution in transition metal complexes are reviewed with particular emphasis on recent developments. Strong support is available for a dissociative mechanism in formation reactions, in which the release of water from the metal ion controls the rate of entry of ligand. This generalization breaks down in the case of certain protonated ligands as well as multidentate ligands of special structure. Some substitution reactions of carbonic anhydrase (in the native and cobalt forms) are reviewed. These include the interaction of enzyme with inhibitors, including sulphonamides and cyanate ion. Kinetic data for these are discussed. The results of studies of the regeneration of the haloenzyme from apoenzyme and metal ion or metal complexes are given and discussed. I had the pleasure of helping Dr. Chatt in the organization of the first I.C.C.C. (although it was not recognized as such at that time). It was held in the I.C.I. Research Laboratories at Weiwyn in 1950 and forty people attended from several countries1. In writing this lecture, I have reflected on the advances that have been made in the area of the kinetics of substitution in the 20 years or so since that small first gathering. In 1950 the terms labile and inert had not been coined. Detailed studies of substitution in Co"1 and Pt" had not been undertaken, and no kinetic study of a labile complex had been reported-----indeed, many of the rapid reaction techniques had yet to be developed2. Perhaps it was not surprising that not one paper on kinetics was included in the 1950 programme! The basic patterns of substitution are now fairly well understood, although many of the details are still missing. Replacement of one ligand by another in a complex is an important process which permeates all aspects of coordination chemistry. It plays a role in formation, hydrolysis, polymerization and redox reactions. Catalysis and inhibition in metalloenzyme-promoted reactions and the transport of metal ions through cell membranes also have substitution processes as an important component. There have been a number of recent accounts of substitution in metal complexes28 and so I have restricted this lecture to a few topics, governed mainly by personal interests and largely concerned with the behaviour in aqueous solution of labile metal complexes, often those of nickel. Some recent chemistry of replacement reactions of carbonic anhydrase which has