Chemistry Q62 – JEE-MAIN 2025

Home Chemistry Coordination CompoundsQuestion #349

An octahedral complex having molecular composition

C o .5 N H_{3} . C l . S O_{4}

has two isomers A and B. The solution of A gives a white precipitate with

A g N O_{3}

solution and the solution of B gives white precipitate with

B a C l_{2}

solution. The type of isomerism exhibited by the complex is,

Clue 1 of 3: Identify the type of reaction

The formation of precipitates with $A g N O_{3}$ and $B a C l_{2}$ indicates the presence of specific free ions in solution.

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Ninja Strategy•Reactivity of Counter Ions

The differential reactivity of isomers A and B with $A g N O_{3}$ and $B a C l_{2}$ directly indicates that different ions are acting as counter ions, which is the defining characteristic of ionisation isomerism.

🧠 Full Solution Path

Step 1: Determine the structure of Isomer A✦ Active

Isomer A gives a white precipitate with $A g N O_{3}$ solution, indicating the presence of free $C l^{-}$ ions in the solution. Therefore, $C l^{-}$ must be outside the coordination sphere. Given the molecular formula $C o .5 N H_{3} . C l . S O_{4}$ and an octahedral complex (coordination number 6), the structure of Isomer A is $[C o (N H_{3})_{5} S O_{4}] C l$ . The oxidation state of $C o$ is $+ 3$ ( $x + 5 (0) + (- 2) = + 1 ⟹ x = + 3$ ).

Step 2: Determine the structure of Isomer B○ Expand

Isomer B gives a white precipitate with $B a C l_{2}$ solution, indicating the presence of free $S O_{4}^{2 -}$ ions in the solution. Therefore, $S O_{4}^{2 -}$ must be outside the coordination sphere. The structure of Isomer B is $[C o (N H_{3})_{5} C l] S O_{4}$ . The oxidation state of $C o$ is $+ 3$ ( $x + 5 (0) + (- 1) = + 2 ⟹ x = + 3$ ).

Step 3: Identify the type of isomerism○ Expand

Isomer A is $[C o (N H_{3})_{5} S O_{4}] C l$ and Isomer B is $[C o (N H_{3})_{5} C l] S O_{4}$ . These two isomers have the same molecular formula but differ in the ions that are inside the coordination sphere and those that are outside as counter ions. Specifically, $C l^{-}$ and $S O_{4}^{2 -}$ exchange positions between the coordination sphere and the ionization sphere. This phenomenon is known as ionisation isomerism.

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Chemistry Question 62 – JEE-MAIN 2025

Clue 1 of 3: Identify the type of reaction

🧠 Full Solution Path