A Closer Look at Substitution Reactions

As has already been discussed, ligands function as Lewis bases when they donate pairs of electrons to a metal ion, which behaves as a Lewis acid, and the coordinate bonds are the result of electron pair donation and acceptance. When NH3 is added to a solution containing [Cu(H20)4]2+, the color changes to a dark blue because of the formation of [Cu(NH3)4]2+ as a result of the substitution of NH3 molecules for H20 molecules. In fact, the most frequently encountered type of reaction of complexes is the substitution of one ligand for another. 

Lewis bases are called nucleophiles because of their attraction to a center of positive charge, and a positive species that attracts a pair of electrons is called an electrophile (see Chapter 5). Therefore, when one Lewis base replaces another, the reaction is called a nucleophilic substitution. In some cases, reactions of this type may be designated as SnI or Sn2 depending on whether the reaction is first-order or second-order, respectively. 

A nucleophilic substitution reaction involving a complex can be shown as 

Substitution reactions are also known in which one metal displaces another as a result of the difference in their strengths as Lewis acids. A general reaction of this type can be represented by the equation 

Because metal ions are electrophiles, a reaction of this type is known as electrophilic substitution (the mechanism may be described as SE1 or SE2). In this reaction, the ligands are transferred from one metal to another, which is equivalent to one metal replacing the other. This type of reaction is much less common than the nucleophilic substitution reactions that will be considered in this section. 

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