Which type of isomerism is shown by the complex compounds?
\([Co(NH_3)_5Br]SO_4\) and \([Co(NH_3)_5SO_4]Br\)
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Ionization isomerism
\( \ce{[Co(NH3)5Br]SO4} \) has \( \ce{Br^{-}} \) as a ligand and \( \ce{SO4^{2-}} \) as the counter ion; \( \ce{[Co(NH3)5SO4]Br} \) has \( \ce{SO4^{2-}} \) as a ligand and \( \ce{Br^{-}} \) as the counter ion. The first gives a white precipitate of \( \ce{BaSO4} \) with \( \ce{BaCl2} \) solution; the second gives a pale yellow precipitate of AgBr with \( \ce{AgNO3} \) solution. They are ionisation isomers.
Why the other options are incorrect
Option (1) Optical isomerism — Incorrect. Needs a chiral complex that is not superimposable on its mirror image; these are two different compounds, not mirror images.
Option (2) Linkage isomerism — Incorrect. Needs an ambidentate ligand bound through different atoms, such as –\( \ce{NO2} \) and –ONO; here a ligand and a counter ion change places.
Option (3) Coordination isomerism — Incorrect. Needs both cation and anion to be complex ions that exchange ligands, as in \( \ce{[Co(NH3)6][Cr(CN)6]} \) and \( \ce{[Cr(NH3)6][Co(CN)6]} \); here only the cation is a complex.
NCERT (2026-27 edition):
Ionisation Isomerism
