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Sphere of Metal Complexes and its Role in Catalysis D. R. EATON McMaster University, Hamilton, Ontario, Canada
Catalysis by transition metal complexes and metalloenzymes involves a sequence of ligand exchange reactions. Such a reaction can be viewed as an interchange of an inner sphere and outer sphere ligand. It is argued therefore that the structure of the second coordination sphere is of direct relevance to catalysis. A method of studying second coordination sphere structure based on dipolar NMR shifts in paramagnetic complexes is discussed. Even in weakly bound complexes, there is a definite preferred structure for the complex which may or may not be favorable for a subsequent substitution reaction.
A lthough the validity of specific models for specific enzyme-catalyzed reactions may be open to question, the most general physical chemical principles are likely to be common to both enzymatic and nonenzymatic catalysis. I would therefore like to make a plea for the consideration of the significance of such a general phenomenon to catalysis, both b y metalloenzymes and i n nonbiological systems. I refer to the structure of the so-called second coordination sphere of metal complexes and its relevance to catalysis. This is an aspect of the field which has not been too widely discussed previously, either at the present symposium or elsewhere. Many of the characteristics of a typical homogeneously catalyzed reaction are, I believe, equally applicable to enzymatic reactions. A suitable example is provided b y the hydroformylation reaction catalyzed by 174 Dessy et al.; Bioinorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
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Second Coordination Sphere
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cobalt carbonyl hydride. The mechanism of this reaction is relatively well understood and may be represented b y the series of steps: H C o ( C O )
