Isomerism shown by the complex pair \( [Co(NH_3)_5(NO_3)]SO_4 \) and \( [Co(NH_3)_5(SO_4)]NO_3 \) is ______
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ionization isomerism
Ionisation isomerism arises when the counter ion of a complex salt can itself act as a ligand and changes place with a ligand inside the coordination sphere. The isomers have the same formula but give different ions in solution.
In this pair the nitrate and sulphate ions have exchanged places:
\( \displaystyle \ce{[Co(NH3)5(NO3)]SO4 -> [Co(NH3)5(NO3)]^2+ + SO4^2-} \)
\( \displaystyle \ce{[Co(NH3)5(SO4)]NO3 -> [Co(NH3)5(SO4)]+ + NO3-} \)
The first gives free sulphate ions (a white precipitate with \( \ce{BaCl2} \)); the second gives free nitrate ions and no precipitate with \( \ce{BaCl2} \). So the pair shows ionisation isomerism.
Why the other options are incorrect
Option (1) Hydrate isomerism — Incorrect. Here water is the species that is inside the sphere in one isomer and free in the crystal in the other, as in \( \ce{[Cr(H2O)6]Cl3} \) and \( \ce{[Cr(H2O)5Cl]Cl2.H2O} \).
Option (3) Coordination isomerism — Incorrect. It needs both cation and anion to be complexes, with ligands exchanged between the two metals, as in \( \ce{[Co(NH3)6][Cr(CN)6]} \) and \( \ce{[Cr(NH3)6][Co(CN)6]} \).
Option (4) Linkage isomerism — Incorrect. It arises from an ambidentate ligand bonding through different atoms, such as \( \ce{NO2-} \) through N (nitrito-N) or O (nitrito-O); the counter ions do not change.
NCERT (2026-27 edition):
Ionisation Isomerism
