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MCQEasyJEE 2023Isomerism in Coordination Compounds

JEE Chemistry 2023 Question with Solution

The complex cation which has two isomers is:

  • A

    [Co(H2O)6]3+[\text{Co}(\text{H}_2\text{O})_6]^{3+}

  • B

    [Co(NH3)5Cl]2+[\text{Co}(\text{NH}_3)_5\text{Cl}]^{2+}

  • C

    [Co(NH3)5NO2]2+[\text{Co}(\text{NH}_3)_5\text{NO}_2]^{2+}

  • D

    [Co(NH3)5Cl]+[\text{Co}(\text{NH}_3)_5\text{Cl}]^{+}

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: We need to identify the complex cation that has two isomers.

Find: Which given complex shows isomerism and specifically gives two isomers.

Analyze each complex:

  1. [Co(H2O)6]3+[\text{Co}(\text{H}_2\text{O})_6]^{3+} All ligands are identical, so only one arrangement is possible. Hence, it does not show isomerism.

  2. [Co(NH3)5Cl]2+[\text{Co}(\text{NH}_3)_5\text{Cl}]^{2+} With only one coordinated Cl\text{Cl} ligand and no suitable counter ion involved in exchange, this does not show the required isomerism here.

  3. [Co(NH3)5NO2]2+[\text{Co}(\text{NH}_3)_5\text{NO}_2]^{2+} The ligand NO2−\text{NO}_2^- is an ambidentate ligand. It can coordinate through nitrogen to give the nitro form, or through oxygen to give the nitrito form. Therefore, this complex shows linkage isomerism and has two isomers.

  4. [Co(NH3)5Cl]+[\text{Co}(\text{NH}_3)_5\text{Cl}]^{+} This also does not show the required isomerism in the given form.

Therefore, the correct option is C.

Linkage Isomerism Explanation

Given: The options are coordination complexes.

Find: The complex cation that exists in exactly two isomeric forms.

The key concept is linkage isomerism. This occurs when a ligand can attach to the central metal atom through two different donor atoms.

Among the given ligands, NO2−\text{NO}_2^- can bind in two ways:

M−NO2(nitro form, bonding through N)M−ONO(nitrito form, bonding through O)\begin{aligned} \text{M}-\text{NO}_2 &\quad \text{(nitro form, bonding through N)} \\ \text{M}-\text{ONO} &\quad \text{(nitrito form, bonding through O)} \end{aligned}

So, [Co(NH3)5NO2]2+[\text{Co}(\text{NH}_3)_5\text{NO}_2]^{2+} exists as two linkage isomers.

By contrast, [Co(H2O)6]3+[\text{Co}(\text{H}_2\text{O})_6]^{3+} has only identical ligands, and the chloride-containing complexes listed here do not generate two linkage isomers.

Hence, the required complex is [Co(NH3)5NO2]2+[\text{Co}(\text{NH}_3)_5\text{NO}_2]^{2+}, so the correct option is C.

Common mistakes

  • Confusing linkage isomerism with ionization isomerism. Here the important point is that NO2−\text{NO}_2^- is an ambidentate ligand and can bond through two different atoms. Do not look for exchange with a counter ion; instead, check the donor atom of the same ligand.

  • Assuming that any complex with different ligands must show isomerism. A complex shows isomerism only when more than one distinct arrangement or mode of bonding is possible. For [Co(NH3)5Cl]2+[\text{Co}(\text{NH}_3)_5\text{Cl}]^{2+}, that condition is not satisfied here.

  • Missing the special behavior of NO2−\text{NO}_2^-. It is not just a single fixed ligand; it can coordinate through N or O. Always identify ambidentate ligands first when checking for linkage isomerism.

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