Hydration isomerism

In coordination chemistry, hydration isomerism is a kind of isomerism that is observed in some solids. Hydration isomers have identical formula but differ with respect to the numbers of water ligands.

Examples
One example is the pair [CrCl(H2O)5]Cl2•H2O and [Cr(H2O)6]Cl3. The former has one water of crystallization but the latter does not.

Another example is the pair of titanium(III) chlorides, [Ti(H2O)6]Cl3 and [Ti(H2O)4Cl2]Cl(H2O)2. The former is violet and the latter, with two molecules of water of crystallization, is green.