Replacement of counterions in ionic complexes

The replacement of one counterion by smother in an ionic complex represents a convenient way to modify the solubility of the compound (see Chapter 2). Often this approach is more convenient dian modification of the structure of a coordinated ligand in order to alter the solubility properties of a complex. Ion exchange thus represents a method by which solubility may be modified. Such reactions are performed in aqueous media, so that a sparingly soluble complex precipitates from solution, or in non-aqueous media, so that a sparingly soluble by-product, usually a salt, precipitates from solution. 

The addition of tetra(n-butyl)ammonium chloride to an aqueous solution of K[PtCl3(MeCN)l results in cation exchange and precipitation of [(M-Bu)4N][PtCl3(MeCN)] with KCl remaining in solution. The addition of sodium perchlorate to aqueous [Fe(bipy)3]Cl2 results in anion exchange and precipitation of [Fe(bipy)3][C104]2 with NaCl remaining in solution. 

In order to exchange sodium cations for cesium in Na2[Fe(CN)s(NO)], a different two-step process is employed. 03 pirst, Na2[Fe(CN)5(NO)] is treated with AgNOs in aqueous solution to produce the sparingly soluble silver salt, Ag2[Fe(CN)5(NO)], and leave NaN03 in solution. Second, treatment of Ag2[Fe(CN)5(NO)] with cesium chloride leaves silver chloride as an insoluble 

In order to generate the barium salt of the titanium tris(catecholate) dianion from its ammonium salt, an acid-base reaction with barium hydroxide has been employed, Eq. 5.26  

Gutmann, Coordination Chemistry in Non-aqueous Solutions (Springer-Verlag, New York, 1968) p. 23. 

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