Ligand substitution by ion exchange in non-aqueous solvents

One successful strategy for ligand substitution by ion exchange involves the use of halide or pseudohalide metal complexes as starting materials in reactions with silver salts that contain the anion that is to be introduced as a new ligand. Since the solubility of silver halides in nonpolar, aprotic solvents is low (see Table 3.1), ion exchange in solvents of poor donor ability typically occurs according to Eq. 5.1  

Because carboxylate anions are hard Lewis bases, they have low affinities for soft metal ions such as platinum(II). Conversely, the relatively soft bromide ion has a high affinity for this metal and yet halide abstraction by Ag+, with concomitant formation of the sparingly soluble salt, AgBr, results in substitution of bromide by the carboxylate anion. The use of dichloromethane, a poor ligand for platinum(II), as the solvent for this reaction prevents any competitive formation of a solvento-complex, although such species may be involved as short-lived intermediates in the reaction. 

A major application of ion exchange in non-aqueous solvents is for the synthesis of organometallic compounds. The common methods fall into two distinct classes. In one, the reagents are electrophilic carbon compounds, such as alkyl halides, and nucleophilic metal complexes, such as carbonylmetallates. In the other, the relative polarities of the reagents are reversed and nucleophilic carbon compounds, e.g. alkyllithium reagents, and electrophilic metal complexes, such as metal halides, are employed. Both classes of reaction are ion exchange processes and typically involve the formation of a salt as the low-solubility by-product. 

Mononuclear metal-alkyl complexes with long-chain alkyl groups have been prepared from sodium carbonylmetallates and alkyl halides, Eq. 5.2  

Similar reactions with a,(o-dihaloalkanes yield binuclear alkanediyl complexes, Eq. 5.3  

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