Incoordination chemistry,hydration isomerism is a kind ofisomerism that is observed in some solids. Hydration isomers have identical formula but differ with respect to the numbers ofwater ligands.
One example is the pair[CrCl(H2O)5]Cl2•H2O and [Cr(H2O)6]Cl3.[1] The former has onewater of crystallization but the latter does not.
Another example is the pair oftitanium(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]