A New Partner - Substitution

We shall examine in some detail here one of the most important, simplest and best understood classes of reactions in coordination chemistry, namely substitution. This involves one (or several) ligand(s) departing the coordination sphere to be replaced by one (or several) others, without a change in coordination number and basic shape between the reactant complex and its product. It is usually a clearly observable reaction, at least where the nature of the ligands involved in the substitution are distinctively different, and can often be followed by simple approaches such as observing the change of colour as reaction proceeds. 

In considering how a substitution can occur, there are really a very limited number of sensible options, which are  

In all cases, regeneration of the preferred coordination number and geometry in forming 

In the above options, (a) involves ligand loss (or dissociation) as the key initiation step, whereas (b) involves ligand gain (or association) as the key initiation step. The third option (c) involves concerted replacement (or interchange) as the key step. As a consequence, they tend to be referred to as dissociative (abbreviated as D), associative (A) and interchange (I) mechanisms in turn. 

The concepts rely also on our understanding of preferred coordination numbers and geometries. For example, for many metal ions six-coordination is common, but some stable examples of both five- and seven-coordination are known. Therefore, it isn t too much of a jump to consider a short-lived five- or seven-coordinate species existing for a species stable only in six-coordination - in other words, a transition state of lower or higher coordination number. As an example, six-coordinate octahedral is the overwhelmingly dominant coordination mode for stable cobalt(ni) complexes. Yet, in recent years, rare examples of isolable but usually very reactive compounds with five-coordination and seven-coordination have been prepared. It doesn t take too much of an act of faith to assert that such geometries form as short-lived transition state species in substitution reactions of this whole family of complexes. 

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