Which type of isomerism is shown by the complexes [Cr(NH3)5Br]Cl and [Cr(NH3)5Cl]Br?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ionisation isomerism
The two complexes have the same formula, but the bromide and chloride ions exchange places between the coordination sphere and the counter-ion position:
\( \ce{[Cr(NH3)5Br]Cl} \) — \( \ce{Br^-} \) is the ligand and \( \ce{Cl^-} \) is the counter ion; in water it gives \( \ce{[Cr(NH3)5Br]^2+ + Cl^-} \).
\( \ce{[Cr(NH3)5Cl]Br} \) — \( \ce{Cl^-} \) is the ligand and \( \ce{Br^-} \) is the counter ion; in water it gives \( \ce{[Cr(NH3)5Cl]^2+ + Br^-} \).
The two compounds give different ions in solution, so they are ionisation isomers.
Why the other options are incorrect
Option (1) Linkage isomerism — Incorrect. Linkage isomerism needs an ambidentate ligand, such as \( \ce{NO2^-} \) or \( \ce{SCN^-} \), bound through different donor atoms; \( \ce{Br^-} \) and \( \ce{Cl^-} \) have only one donor atom each.
Option (3) Hydrate isomerism — Incorrect. Hydrate isomers differ in whether a water molecule is bonded to the metal or is free in the crystal lattice; no water is present here.
Option (4) Coordination isomerism — Incorrect. Coordination isomerism needs two complex ions (a cationic and an anionic entity of different metals) exchanging ligands; each compound here has only one complex ion.
NCERT (2026-27 edition):
5.4.5 Ionisation Isomerism – “This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4.”