\([CuSO_4(NH_3)_2] Br_2\) and \([Cu(NH_3)_2Br_2]SO_4\) are:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Ionisation isomers
The two compounds have the same composition, but the ion that is a ligand in one is the counter ion in the other:
\( \ce{[CuSO4(NH3)2]Br2} \) — SO42− is a ligand, \( \ce{Br^{-}} \) ions are outside the coordination sphere; in solution it gives \( \ce{2Br^{-}} \) ions.
\( \ce{[Cu(NH3)2Br2]SO4} \) — Br− ions are ligands, \( \ce{SO4^{2-}} \) is the counter ion; in solution it gives \( \ce{SO4^{2-}} \) ions.
Such compounds give different ions in solution and are ionisation isomers.
Why the other options are incorrect
Option (1) Coordination isomers — Incorrect. These arise from the interchange of ligands between a complex cation and a complex anion, e.g. \( \ce{[Co(NH3)6][Cr(CN)6]} \) and \( \ce{[Cr(NH3)6][Co(CN)6]} \); here there is only one complex ion.
Option (2) Linkage isomers — Incorrect. These need an ambidentate ligand bonded through different atoms (–\( \ce{NO2} \)/–ONO, –SCN/–NCS); no such change is involved here.
Option (3) Geometrical isomers — Incorrect. These have the same ligands arranged differently in space (cis/trans); here the ligands themselves are different.
NCERT (2026-27 edition):
Ionisation Isomerism
