A C&EN Feature

O f the 103 known elements, 83 are classified as metals. Much of the chemistry of this very large and diverse family of elements is the chemistry of metals as ions in solution. This includes some of those most familiar of all chemical reactions that we meet in our first introduction to chemistry: the precipitation of halides as their silver salts for identification; the changes of color of copper ion solution as ammonia is added; and many more. Metal ions in solution are always bound to other molecules or negative ions. Species capable of good binding to metal ions are referred to as ligands and may be solvent molecules or other good coordinating groups. The metal plus the most firmly bound ligands make up an aggregate species called an inner sphere complex. Species immediately outside the inner sphere complex are in the "outer sphere" environment. Such complex compounds of metals play key roles in important chemical processes. Hemoglobin, the oxygen carrier of blood, is an iron complex. The green pigment of plants, chlorophyll, is a similar compound of magnesium. Vitamin B12 contains a cobalt ion bound to another elaborate ligand system. Many catalysts important to chemical synthesis are metal ion complexes, and metal complexes intervene in most electroplating processes. If we are to understand and control the chemistry of metal ions in solution, we must understand the processes by which their complexes are formed and in ter con verted. This is most often a matter of the replacement of one coordinated ligand by another. The substitution of a ligand in the inner sphere (leaving group) by an outside group (entering group) is the most fundamental of the reactions in metal ion chemistry. In fact, all discussions of reactions of metal complexes hinge on an understanding of the dynamics of their ligand substitution process, since it is an ever present possibility. To attempt a general theory of ligand substitution dynamics seems a formidable task, since so many elements of such diverse properties fall within the scope of the undertaking. However, substantial progress has been made, especially in recent years, because of a fascinating convergence of: • Results of studies of certain model systems by traditional methods for analysis of rate and mechanism. • Results over a broad time range made available by application of new fast reaction techniques developed largely by Prof. Manfred Eigen and his collaborators in West Germany. • Results of applications of improved electronic structural models for metal complexes to give added power to theoretical correlations. In this article we outline some recent advances in the understanding of ligand substitution dynamics to reveal the effectiveness of the interplay of new and old techniques with fresh theoretical insights.