[Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O are an example of which of the following types of isomerism?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Hydrate isomerism
Both compounds have the formula \( \ce{CrCl3.6H2O} \).
In \( \ce{[Cr(H2O)6]Cl3} \) all six water molecules are bonded to Cr; in \( \ce{[Cr(H2O)5Cl]Cl2.H2O} \) one water molecule has left the coordination sphere for a \( \ce{Cl^-} \) and sits as free water in the crystal lattice.
Isomers that differ in whether the solvent is bonded to the metal or free are solvate isomers; with water as the solvent this is hydrate isomerism.
Why the other options are incorrect
Option (1) Linkage isomerism — Incorrect. It needs an ambidentate ligand (such as \( \ce{NO2^-} \) or \( \ce{SCN^-} \)) bound through different atoms; neither water nor chloride is ambidentate.
Option (3) Ionisation isomerism — Incorrect. In ionisation isomers a counter ion swaps places with a ligand, as in \( \ce{[Co(NH3)5(SO4)]Br} \) and \( \ce{[Co(NH3)5Br]SO4} \); here the change concerns where a solvent water molecule sits, which is named separately as hydrate isomerism.
Option (4) Coordination isomerism — Incorrect. It needs two complex ions (a cationic and an anionic entity of different metals) exchanging ligands; these compounds contain only one metal and one complex ion.
NCERT (2026-27 edition):
5.4.6 Solvate Isomerism – “This form of isomerism is known as ‘hydrate isomerism’ in case where water is involved as a solvent. … Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice. An example is provided by the aqua complex [Cr(H2O)6]Cl3 (violet) and its solvate isomer [Cr(H2O)5Cl]Cl2.H2O (grey-green).”