The compounds \( \ce{[Co(NH3)5Br]SO4} \) and \( \ce{[Co(NH3)5(SO4)]Br} \) are dissolved separately in water, and each solution is tested with barium chloride solution and with silver nitrate solution. Which of the following observations is correct?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( \ce{[Co(NH3)5(SO4)]Br} \) gives a precipitate of AgBr with silver nitrate solution.
In ionisation isomers a ligand and the counter ion change places. The two compounds here are ionisation isomers. Only the ion outside the square bracket (the counter ion) is set free in water; the ion bonded to cobalt inside the bracket does not react with the test reagent.
In \( \ce{[Co(NH3)5(SO4)]Br} \) the bromide ion is the counter ion, so silver nitrate precipitates silver bromide:
\( \displaystyle \ce{[Co(NH3)5(SO4)]Br + AgNO3 -> [Co(NH3)5(SO4)]NO3 + AgBr v} \)
In \( \ce{[Co(NH3)5Br]SO4} \) the sulphate ion is the counter ion, so barium chloride precipitates barium sulphate:
\( \displaystyle \ce{[Co(NH3)5Br]SO4 + BaCl2 -> [Co(NH3)5Br]Cl2 + BaSO4 v} \)
Why the other options are incorrect
Option (1) \( \ce{[Co(NH3)5Br]SO4} \) gives a precipitate of AgBr with silver nitrate solution. — Incorrect. In \( \ce{[Co(NH3)5Br]SO4} \) the bromide is a ligand bonded to cobalt, so it is not free to give AgBr; this isomer shows no reaction with Ag+.
Option (2) \( \ce{[Co(NH3)5(SO4)]Br} \) gives a precipitate of BaSO4 with barium chloride solution. — Incorrect. In \( \ce{[Co(NH3)5(SO4)]Br} \) the sulphate is inside the coordination sphere, so it does not give BaSO4; this isomer shows no reaction with Ba2+.
Option (3) \( \ce{[Co(NH3)5Br]SO4} \) gives no precipitate with barium chloride solution. — Incorrect. \( \ce{[Co(NH3)5Br]SO4} \) has a free sulphate ion, so it does give a precipitate of BaSO4 with barium chloride, as in the second equation above.
NCERT (2026-27 edition):
5.4.5 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.”
Answers to Some Intext Questions – “5.4 The ionisation isomers dissolve in water to yield different ions and thus react differently to various reagents:
[Co(NH3)5Br]SO4 + Ba2+ → BaSO4 (s)
[Co(NH3)5SO4]Br + Ba2+ → No reaction
[Co(NH3)5Br]SO4 + Ag+ → No reaction
[Co(NH3)5SO4]Br + Ag+ → AgBr (s)”