Two isomeric compounds, \(A: [Co(NH_3)_5Cl]SO_4\) and \(B: [Co(NH_3)_5SO_4]Cl\) can be distinguished using which of the following?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → aq. BaCl2 solution
The two compounds are ionisation isomers: they exchange the ligand inside the coordination sphere with the counter ion outside it, so they give different ions in water:
A: \( \ce{[Co(NH3)5Cl]SO4} \) → \( \ce{[Co(NH3)5Cl]^{2+}} \) + SO42−
B: \( \ce{[Co(NH3)5SO4]Cl} \) → \( \ce{[Co(NH3)5SO4]^{+}} \) + Cl−
With aqueous \( \ce{BaCl2} \), A gives a white precipitate of BaSO4 (\( \ce{Ba^{2+}} \) + \( \ce{SO4^{2-}} \) → \( \ce{BaSO4} \)↓). B gives none, because its sulphate is bound to cobalt and is not free in solution.
Why the other options are incorrect
Option (2) Nessler’s reagent — Incorrect. It tests for ammonia/ammonium ions; both compounds have the same \( \ce{NH3} \) ligands, and these are held in the complex.
Option (3) Ceric ammonium nitrate reagent — Incorrect. It is a test for alcohols and phenols and does not tell sulphate from chloride.
Option (4) Passing H2S gas through their aqueous solution — Incorrect. Both contain the same metal, Co(III), held in the complex, so \( \ce{H2S} \) cannot tell them apart.
NCERT (2026-27 edition):
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.”