Solubility of complexes in mixed solvents

In contrast, mixed solvent systems are used widely in preparative coordination chemistry for the precipitation of compounds from solution. This operation is based on the compound s decreased solubility in the mixed solvent. 

An increase in the solubility of a complex is anticipated when the complex has acid properties and the solvent, or one of its components, has basic properties. For example, the increased solubility of inert amine complexes of transition metals in liquid ammonia or amines is known Complexes with basic properties are possible in principle and, although they are encountered only rarely in practice, such complexes should have increased solubility in acidic solvents. 

The formation of outer-sphere complexes also alters the solubilities of compounds. The solubility method is one of the few techniques which can be used to study outer-sphere complexes and their stepwise formation. For example, addition of perchlorate ion to aqueous solutions of hexamminecobalt(III) perchlorate first decreases the solubility, according to the law of mass action, but then, at higher concentrations of perchlorate, the solubility increases much more than changes in ionic strength would require.These experiments were used to demonstrate the ability of perchlorate ions to form ion pairs with the cobalt(III) cations and to determine the association constant. 

In conclusion, complexes undergo interactions with solvent molecules forming new species in solution. Such interactions can be detected by observation of increased solubility. The potential for such interactions must be carefully considered when a solvent is selected for a given metal complex. 

Eggins, Chemical Structure and Reactivity (MacMillan, London, 1972) p. 140. 

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