Dear friend,

Solvate or hydrate isomerism are the isomers which have the same composition but differ with respect to the number of solvent ligand molecules as well as the counter ion in the crystal lattice.
For example [Cr(H2O)6]Cl3 is violet colored, [CrCl(H2O)5]Cl2H2O is blue-green, and [CrCl2(H2O)4]Cl2H2O is dark green.
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 calledHydrate isomerism.
Hope this may help u .