JEE Chemistry 2025 Question with Solution

Given: The complexes are $[Co(NH_3)_3Cl_3]$, $[Co(NH_3)_4Cl_2]^+$, $[Co(en)_3]^{3+}$ and $[Co(en)_2Cl_2]^+$.

Find: Which complex shows facial-meridional isomerism.

Facial (fac) and meridional (mer) isomers are geometric isomers found in octahedral complexes of the general formula $[MA_3B_3]$.

In such complexes, three identical ligands can be arranged in two ways:

1. Facial: the three identical ligands occupy adjacent positions and form one face of the octahedron.
2. Meridional: the three identical ligands lie in a plane along a meridian of the octahedron.

Now examine the options:

• $[Co(NH_3)_3Cl_3]$ has the form $[MA_3B_3]$, so it can show fac-mer isomerism.
• $[Co(NH_3)_4Cl_2]^+$ has the form $[MA_4B_2]$, which shows cis-trans isomerism, not fac-mer isomerism.
• $[Co(en)_3]^{3+}$ contains three bidentate ligands, so fac-mer isomerism does not arise.
• $[Co(en)_2Cl_2]^+$ has two chloride ligands and two bidentate ligands, so it does not satisfy the $[MA_3B_3]$ requirement.

Therefore, the correct complex exhibiting facial-meridional isomerism is $[Co(NH_3)_3Cl_3]$.

The correct option is A.

Given: Facial-meridional isomerism occurs in octahedral complexes when three identical ligands can occupy either one face or one meridian.

Find: Which listed complex satisfies this condition.

For fac-mer isomerism, the key structural requirement is an octahedral complex of type $[MA_3B_3]$.

• In $[Co(NH_3)_3Cl_3]$, there are three identical $NH_3$ ligands and three identical $Cl^-$ ligands. Hence both fac and mer arrangements are possible.
• In $[Co(NH_3)_4Cl_2]^+$, only two chloride ligands are present, so fac-mer arrangement is not possible.
• In $[Co(en)_3]^{3+}$, the ligand $en$ is bidentate and the complex is homoleptic; this does not generate fac-mer isomerism.
• In $[Co(en)_2Cl_2]^+$, there are only two chloride ligands and two bidentate ligands, so the required three identical monodentate ligands are absent.

Hence only $[Co(NH_3)_3Cl_3]$ shows facial-meridional isomerism.

The correct option is A.