Question

Explain and illustrate with an example of the following in relation to coordination compounds:

Hydrate isomerism

Answer 1

The water molecules can act both as ligands as well as simple molecules of water of crystallisation. When two complexes differ in this aspect, they exhibit hydrate isomerism. For example, with the formula CrCl₃·6H₂O, the following three isomers are known to exist.

1. [Cr(H₂O)₆]Cl₃ - Violet
   hexaaquachromium (III) chloride 
2. [Cr(H₂O)₅Cl]Cl₂.2H₂O - Blue-green
   pentaaquachloridochromium (III) chloride monohydrate
3. [Cr(H₂O)₅Cl₂]Cl.2H₂O - Dark green
   traaquadichloridochromium (III) chloride dihydrate

The quantity of water molecules bonded to the core chromium atom varies between these isomers. They can be further distinguished by conductivity tests in addition to having distinct hues.