Which of the following compounds would exhibit coordination isomerism?

The correct answer is Option (2) → [Pt(NH₃)₄][CuCl₄]

Coordination isomerism needs a compound in which both the cation and the anion are complex entities of different metal ions; the isomers differ in which ligands sit on which metal.

In \( \ce{[Pt(NH3)4][CuCl4]} \) the \( \ce{NH3} \) ligands are bound to \( \ce{Pt^2+} \) and the \( \ce{Cl^-} \) ligands to \( \ce{Cu^2+} \). Interchanging them gives its coordination isomer:

    \( \displaystyle \ce{[Pt(NH3)4]^2+[CuCl4]^2-} \quad \text{and} \quad \ce{[Cu(NH3)4]^2+[PtCl4]^2-} \)

Why the other options are incorrect

Option (1) [Cr(H₂O)₆]Cl₃ — Incorrect. It has only one complex entity, the cation; the \( \ce{Cl^-} \) ions are simple counter ions. It shows hydrate isomerism instead, with \( \ce{[Cr(H2O)5Cl]Cl2.H2O} \).

Option (3) [Cr(en)₂Cl₂]NO₂ — Incorrect. Again one complex cation with a simple anion, so no ligand exchange between two metals is possible. As an [MX₂(L–L)₂] complex it shows geometrical (cis-trans) isomerism.

Option (4) [Co(NH₃)₅Br]SO₄ — Incorrect. It has one complex cation; the counter ion \( \ce{SO4^2-} \) can swap places with \( \ce{Br^-} \), giving ionisation isomerism with \( \ce{[Co(NH3)5(SO4)]Br} \).

NCERT (2026-27 edition):
5.4.4 Coordination Isomerism – “This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex. An example is provided by [Co(NH₃)₆][Cr(CN)₆], in which the NH₃ ligands are bound to Co³⁺ and the CN^– ligands to Cr³⁺. In its coordination isomer [Cr(NH₃)₆][Co(CN)₆], the NH₃ ligands are bound to Cr³⁺ and the CN^– ligands to Co³⁺.”