Solvate Isomerism

Solvate isomerism is closely related to ionisation isomerism, and takes the specific name hydrate isomerism in the particular case where the solvent involved is water.

Solvate isomers differ according to whether or not a solvent molecule is directly bonded to the metal ion — that is, whether it occupies a coordination position as a ligand — or is instead present merely as free solvent molecules of crystallisation within the crystal lattice, outside the coordination sphere.

Example

An example is provided by the aqua complex $[Cr(H_2O)_6]Cl_3$, which is violet, and its solvate isomer $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$, which is grey-green.

In the first compound all six water molecules are coordinated to the chromium centre, and all three chloride ions are free counter ions. In the second compound, one water molecule has been replaced in the coordination sphere by a chloride ion — so only five water molecules are coordinated, one chloride is now a ligand, and the remaining water molecule sits outside the coordination sphere as a molecule of crystallisation, with the other two chlorides as free counter ions. …