Effect of Counterions. Anion Control

In water and other highly solvating media, the charged complex and the anion are separately solvated, and no anion effect on complex stability is expected. However, in poorly solvating media more or less pronounced 

The electrostatic cation-anion interactions will depend on the properties of the anion its charge, its size, its shape, its polarizability. Large anions lead to weaker interactions because of larger cation-anion distances ion-paired complexes of divalent AEC s will be much more destabilized by an increase in anionic size than AC complexes  

the anion-cation interactions will also be affected by the dielectric constant of the surrounding solvent a decrease in e will increase the strength of these interactions since the electrostatic interactions are proportional to 1/s. 

As has already been pointed out, the solubility properties of the anion, its lipophilicity, are extremely important for the dissolution of the complex in solvents of low polarity. Large and soft inorganic and, much more so, organic anions very strongly increase the solubility. Anion activation and cation transport processes both depend on such anion effects. 

Finally, hydrophobic interactions may also be of importance if the anion is markedly hydrophobic it may have a tendency, in aqueous solution, to associate with the complexed cation, which is much more lipophilic than the free cation. 

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