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 {{chem2|[CrCl(H2O)5]Cl2•H2O and [Cr(H2O)6]Cl3}}.{{cite journal |doi=10.1021/ed049p204 |title=The Hydration Isomers of Chromium(III) Chloride |year=1972 |last1=Barbier |first1=J. P. |last2=Kappenstein |first2=C. |last3=Hugel |first3=R. |journal=Journal of Chemical Education |volume=49 |issue=3 |page=204 |bibcode=1972JChEd..49..204B }} The former has one water of crystallization but the latter does not.
Another example is the pair of titanium(III) chlorides, {{chem2|[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.{{Greenwood&Earnshaw2nd|page = 965}}