Substitution Reactions in Nonaqueous Solvents

Water may not be a suitable solvent in all cases due to reactant insolubility (of the precursor complex and/or the ligand, but usually the latter), extreme inertness that demands use of a higher boiling point solvent for reaction, or high stability of undesired hydroxo or oxo species that prevent or interfere with formation of the desired products. This is usually 

The aqueous chemistry of particularly Al(III), Fe(III) and Cr(III) involves formation of strong M—O bonds, and in basic aqueous solution, hydroxide species (or unreactive oligomers) usually precipitate preferentially and rapidly because many added ligands are strong bases that cause a rise in solution pH. The behaviour of Fe(III) in a weakly basic aqueous solution is a good example, and can be represented simplistically by (6.12). 

Where solubility alone is the issue, simply changing solvent to permit all species to be dissolved allows the chemistry to proceed essentially as it would in aqueous solution were species soluble. Typical molecular organic solvents used in place of water include other protic solvents such as alcohols (e.g. ethanol), and aprotic solvents such as ketones (e.g. acetone), amides (e.g. dimethylformamide), nitriles (e.g. acetonitrile) and sulfoxides (e.g. dimethylsulfoxide). Recently, solvents termed ionic liquids, which are purely ionic material that are liquid at or near room temperature, have been employed for synthesis typically, they consist of a large organic cation and an inorganic anion (e.g. lV, lV,-butyl(methyl)-imidazolium nitrate) and their ionic nature supports dissolution of, particularly, ionic complexes. 

In some case, where hydrolysis reactions are not of concern, mixtures or organic solvents and water can be employed. For example, it is possible to mix an aqueous solution of the 

Some complexes may not form, or else are not stable, in water because they are not thermodynamically stable in that solvent. Their formation in the total absence of water can be achieved, however, as exemplified by the sequence of reactions below where the neutral tetrahedral complex can be isolated readily and is stable in the absence of water, although dissolution in water leads to very rapid formation of the octahedral Co2+aq cation and dissociation of the initial ligands (6.15). 

The aqueous reaction between a chromium(III) salt and ethylenediamine is shown by equation (7). If, instead, an anhydrous chromium salt and a non- 

In some cases, a nonaqueous solvent is required because the ligand is not water soluble. Often it suffices to dissolve the ligand in a water-miscible solvent and then to add this solution to a concentrated water solution of the metal ion. Metal complexes of bipy and phen are generally prepared by this way. Thus the addition of an alcoholic solution of bipy to an aqueous solution of FeCh readily yields the complex [Fe(bipy)3]Cl2, reaction (10). 

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