Structural isomerism: hydration isomers

   Hydration isomers result from the interchange of H₂O and another ligand between the first coordination sphere and the ligands outside it.  The classic example of hydrate isomerism is that of the compound of formula CrCl₃.6H₂O. Green crystals of chromium(III) chloride formed from a hot solution obtained by reducing chromium(VI) oxide with concentrated hydrochloric acid are [Cr(H₂O(₄Cl₂]Cl.2H₂O. When this is dissolved in water, the chloride ions in the complex are slowly replaced by water to give blue-green [Cr(H₂O)₅Cl]Cl₂.H₂O and finally violet [Cr(H₂O)₆]Cl₃.   The complexes can be distinguished by precipitation of the free chloride ion using aqueous silver nitrate.