Ionization isomers are identical except for a ligand has exchanging places with an anion or neutral molecule that was originally outside the coordination complex. The central ion and the other ligands are identical. For example, an octahedral isomer will have five ligands that are identical, but the sixth will differ. The non-matching ligand in one compound will be outside of the coordination sphere of the other compound. Because the anion or molecule outside the coordination sphere is different, the chemical properties of these isomers is different. A hydrate isomer is a specific kind of ionization isomer where a water molecule is one of the molecules that exchanges places.
Example 1: Ionization Isomerism
We have pentaaquabromocobaltate(II)chloride which changes to pentaaquachlorocobaltate(II)bromide.
Ionization isomers: where the isomers can be thought of as occurring because of the formation of different ions in solution.I
one isomer [PtBr(NH3)3]NO2 -> NO2- anions in solution
another isomer [Pt(NO2)(NH3)3]Br -> Br- anions in solution
Notice that both anions are necessary to balance the charge of the complex, and that they differ in that one ion is directly attached to the central metal but the other is not. A very similar type of isomerism results from replacement of a coordinated group by a solvent molecule ("Solvate Isomerism"). In the case of water, this is called Hydrate isomerism. The best known example of this occurs for chromium chloride "CrCl3.6H2O" which may contain 4, 5, or 6 coordinated water molecules.
- \([CrCl_2(H_2O)_4]Cl \cdot 2H_2O\): bright-green colored
- \([CrCl(H_2O)_5]Cl_2 \cdot H_2O\): grey-green colored
- \([Cr(H_2O)_6]Cl_3\): violet colored
These isomers have very different chemical properties and on reaction with \(AgNO_3\) to test for \(Cl^-\) ions, would find 1, 2, and 3 \(Cl^-\) ions in solution respectively.
Figure 1: Ionization Isomers: (left) Hexaamminecobalt(III) Chloride and (right) Pentaamminechlorocobalt (III) Chloride