JEE Chemistry 2025 Question with Solution

Given: An octahedral complex with composition $\mathrm{Co}.5\mathrm{NH}_3.\mathrm{Cl}.\mathrm{SO}_4$ has two isomers A and B. Isomer A gives a white precipitate with AgNO$\_3$, while isomer B gives a white precipitate with BaCl$\_2$.

Find: The type of isomerism exhibited by the complex.

A white precipitate with AgNO$\_3$ indicates free $\mathrm{Cl}^-$ ions in solution, because $\mathrm{Ag}^+ + \mathrm{Cl}^- \rightarrow \mathrm{AgCl} \downarrow$.

A white precipitate with BaCl$\_2$ indicates free $\mathrm{SO}_4^{2-}$ ions in solution, because $\mathrm{Ba}^{2+} + \mathrm{SO}_4^{2-} \rightarrow \mathrm{BaSO}_4 \downarrow$.

Thus, in one isomer chloride ion is outside the coordination sphere, and in the other isomer sulfate ion is outside the coordination sphere.

These may be represented as:

and

The difference between the two compounds is the exchange of an ion inside the coordination sphere with an ion outside the coordination sphere. This is the defining feature of ionisation isomerism.

Therefore, the correct option is C.

Given: The two isomers give different precipitates with AgNO$\_3$ and BaCl$\_2$.

Find: Which isomerism explains this behavior.

If AgNO$\_3$ gives a white precipitate, then $\mathrm{Cl}^-$ must be present as a free ion in solution. If BaCl$\_2$ gives a white precipitate, then $\mathrm{SO}_4^{2-}$ must be present as a free ion in solution.

So the two isomers release different ions on dissolution. They have the same molecular composition, but differ in which ion is coordinated to cobalt and which ion remains outside as counter-ion.

Compounds that have the same molecular formula but produce different ions in solution show ionisation isomerism. Hence, the complex exhibits ionisation isomerism and the correct option is C.