Metal complexes ligand substitution mechanisms

Although quantitative studies of substitution reactions have only been launched quite recently, a considerable body of experimental data is available, so that theoretical principles of ligand substitution mechanisms in classical inorganic complexes have been developed. Unfortunately the same cannot be said for substitution reactions involving 7r-bonded hydrocarbon ligands, in spite of a continuously expanding number of publications in this field. In the majority of studies substitution reactions are used to obtain novel transition metal 7r-complexes, and a far lesser number of papers deal with the quantitative aspects of the exchange. 

Information on ligand substitution mechanisms should aid us to understand more profoundly homogeneous catalysis by transition metal complexes, where probably consecutive substitution and transfer reactions of ligands from metal to a substrate and back take place continuously. 

Previously reported work demonstrated that substituents can be used to tune the energies of excited states responsible for the emission spectra of certain group VIII metal complexes (1) and to modify significantly the absorption spectra of complexes displaying metal-to-ligand charge transfer (MLCT) bands (2). In this presentation, we summarize some recent attempts to use ligand substituents in our studies of transition metal complex photochemical reaction mechanisms. The particular subjects of interest are the metal ammine complexes M(NH3)5L where M is Rh(III) or Ru(II) and L is a meta- or para-substituted pyridine. 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

The chapter on kinetics and mechanisms of complex formation and ligand substitution at alkali metal and alkaline earth cations elsewhere in this volume provides context and complementary discussion of these processes in relation to calcium. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

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