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.[1] 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.[2]
References
^Barbier, J. P.; Kappenstein, C.; Hugel, R. (1972). "The Hydration Isomers of Chromium(III) Chloride". Journal of Chemical Education. 49 (3): 204. Bibcode:1972JChEd..49..204B. doi:10.1021/ed049p204.