Ligand substitution stereochemistry

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

For a study on variation of product stereochemistry with ligand substitution in copper catalyzed reactions see Evans, D. A. Johnson, J. S. Burgey, C. S. Campos, K. R. Tetrahedron Lett. 1999, 40, 2879-2882. 

Neither AG° nor A0 depends upon the mechanism of the reaction. But even if AG° is strongly negative, the yield of thermodynamically favored products may be negligible if the reaction proceeds too slowly on the human timescale or if the slowness of a critical step in the reaction sequence relative to some alternative steers the reaction to other (metastable) products. Thus, as Taube1 emphasized, we need to understand what makes some ligand substitution reactions fast and others slow. The mechanism of reaction is a simplified hypothetical model, an approximation to reality that purports to trace the progress of the system from reactants to products, and is significant only insofar as it helps us understand the kinetics and stereochemistry of the reaction (rather than vice versa as some workers tend to believe). 

Although Ru(III) ammine complexes are known to be very inert low-spin d species which only very slowly undergo substitution reactions, their ability to rapidly and efiectively bind nitric oxide seems to be a rather unusual behavior (92). Common characteristics of the Ru(III) nitrosyl complexes, formally Ru NO, studied to date are their octahedral stereochemistry and the presence of an extremely stable Ru—NO mode (93). A broad array of available kinetic and electrochemical data dealing with the formation of Ru(III) nitrosyls clearly shows that the mechanism of unusual fast coordination of nitric oxide to the Ru(III) ammine center cannot be accounted for in terms of a classical ligand substitution process. In this context, the fundamental kinetics of the fast reactions between [Ru (NH3)5X] pC = Cl, ... 

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