Question:medium
Which of the following coordination complex ions exhibits structural asymmetry leading to optical isomerism?
Which of the following coordination complex ions exhibits structural asymmetry leading to optical isomerism?
Show Hint
Octahedral metal configurations of the general type \([\text{M}(\text{AA})_2\text{X}_2]\) with bidentate chelating rings display optical activity exclusively in their cis arrangements.
Updated On: May 30, 2026
- \(\text{trans}-[\text{Co}(\text{en})_2\text{Cl}_2]^+ \)
- \(\text{cis}-[\text{Co}(\text{en})_2\text{Cl}_2]^+ \)
- \([\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+ \)
- \([\text{Cr}(\text{NH}_3)_5\text{Cl}]^{2+} \)
Show Solution
The Correct Option is B
Solution and Explanation
Step 1 : Understanding the Question:
The topic of this question is Isomerism in Coordination Compounds, specifically focusing on Optical Isomerism. Optical isomers (enantiomers) are molecules that are non-superimposable mirror images of each other. In coordination chemistry, this typically happens when a complex lacks a plane of symmetry or a center of inversion. The question asks us to identify which coordination complex among the choices is chiral and thus optically active.
Step 2 : Key Formulas and approach:
The approach involves checking the symmetry elements of each complex:
1. Identify the geometry: All provided complexes are octahedral (coordination number 6).
2. Check for Plane of Symmetry ($\sigma$): If a plane can divide the molecule into two identical halves, it is achiral (optically inactive).
3. Focus on Chelating Ligands: Optical isomerism is very common in octahedral complexes containing bidentate ligands like ethylenediamine (en).
Step 3 : Detailed Explanation:
Option (A): $trans-[Co(en)_2Cl_2]^+$. In the trans isomer, the two Chlorine ligands are opposite each other (180 degrees), and the two 'en' rings are also opposite. This molecule has a horizontal plane of symmetry passing through the Cobalt and the 'en' ligands. Therefore, it is achiral and optically inactive.
Option (B): $cis-[Co(en)_2Cl_2]^+$. In the cis isomer, the two Chlorine ligands are adjacent (90 degrees), and the 'en' rings are also forced into a specific corner of the octahedron. This arrangement has no plane of symmetry or center of inversion. Consequently, it forms two non-superimposable mirror images (d- and l-forms). It is chiral and exhibits optical isomerism.
Option (C): $[Co(NH_3)_4Cl_2]^+$. This complex has only unidentate ligands. Even its cis-isomer has a plane of symmetry passing through the central metal. Thus, it does not show optical isomerism.
Option (D): $[Cr(NH_3)_5Cl]^{2+}$. This is an $MA_5B$ type complex, which is highly symmetrical and possesses multiple planes of symmetry. It is optically inactive.
Comparing all options, only the cis configuration of a bis-chelating complex provides the necessary asymmetry for chirality.
Step 4 : Final Answer:
The cis isomer of $[Co(en)_2Cl_2]^+$ is the only complex in the list that lacks symmetry and shows optical isomerism. The correct option is (B).
The topic of this question is Isomerism in Coordination Compounds, specifically focusing on Optical Isomerism. Optical isomers (enantiomers) are molecules that are non-superimposable mirror images of each other. In coordination chemistry, this typically happens when a complex lacks a plane of symmetry or a center of inversion. The question asks us to identify which coordination complex among the choices is chiral and thus optically active.
Step 2 : Key Formulas and approach:
The approach involves checking the symmetry elements of each complex:
1. Identify the geometry: All provided complexes are octahedral (coordination number 6).
2. Check for Plane of Symmetry ($\sigma$): If a plane can divide the molecule into two identical halves, it is achiral (optically inactive).
3. Focus on Chelating Ligands: Optical isomerism is very common in octahedral complexes containing bidentate ligands like ethylenediamine (en).
Step 3 : Detailed Explanation:
Option (A): $trans-[Co(en)_2Cl_2]^+$. In the trans isomer, the two Chlorine ligands are opposite each other (180 degrees), and the two 'en' rings are also opposite. This molecule has a horizontal plane of symmetry passing through the Cobalt and the 'en' ligands. Therefore, it is achiral and optically inactive.
Option (B): $cis-[Co(en)_2Cl_2]^+$. In the cis isomer, the two Chlorine ligands are adjacent (90 degrees), and the 'en' rings are also forced into a specific corner of the octahedron. This arrangement has no plane of symmetry or center of inversion. Consequently, it forms two non-superimposable mirror images (d- and l-forms). It is chiral and exhibits optical isomerism.
Option (C): $[Co(NH_3)_4Cl_2]^+$. This complex has only unidentate ligands. Even its cis-isomer has a plane of symmetry passing through the central metal. Thus, it does not show optical isomerism.
Option (D): $[Cr(NH_3)_5Cl]^{2+}$. This is an $MA_5B$ type complex, which is highly symmetrical and possesses multiple planes of symmetry. It is optically inactive.
Comparing all options, only the cis configuration of a bis-chelating complex provides the necessary asymmetry for chirality.
Step 4 : Final Answer:
The cis isomer of $[Co(en)_2Cl_2]^+$ is the only complex in the list that lacks symmetry and shows optical isomerism. The correct option is (B).
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