The Characteristics of Substitution Reactions

Ligand interchange in metal complexes can occur in two ways, either (a) by a combination of solvolysis and ligation e.g.  

Indirect substitution of the type indicated in (4.1) and (4.2) appears to be the method much preferred by octahedral complexes, while direct substitution is more relevant with square-planar complexes. This situation could perhaps be predicted in view of the more crowded conditions with octahedral than with planar complexes. For other geometries both routes are used. 

The substitution process permeates the whole realm of coordination chemistry. It is frequently the first step in a redox reaction and in the dimerization or polymerization of a metal ion, the details of which in many cases are still rather scanty (e.g. for Cr(III) ). An understanding of the kinetics of substitution can be important for defining the best conditions for a preparative or analytical procedure. Substitution pervades the behavior of metal or metal-activated enzymes. The production of apoprotein (demetalloprotein and the regeneration of the protein, as well as the interaction of substrates and inhibitors with metalloproteins are important examples.  

---