Which of the following complex species is not expected to exhibit optical isomerism ?

Options

A. `[Co(NH_3)_3Cl_3]`
B. `[Co(en)(NH_3)_2Cl_2]^+`
C. `[Co(en)_3]^(3+)`
D. `[Co(en)_2Cl_2]^(+)`

Answer

To determine which complex species is not expected to exhibit optical isomerism, we need to analyze the symmetry of each complex. Optical isomerism occurs in complexes that lack a plane of symmetry, meaning they are chiral and can exist in two non-superimposable mirror images.

### Step-by-Step Solution:

1. **Understanding Optical Isomerism**:
- Optical isomerism is exhibited by chiral molecules, which do not have any symmetry elements (such as a plane of symmetry).
- If a complex has a plane of symmetry, it is considered achiral and will not exhibit optical isomerism.

2. **Analyzing the Given Complexes**:
- **Complex 1: [Co(NH3)3Cl3]**
- This complex can form two geometrical isomers: facial (fac) and meridional (mer).
- Both isomers have a plane of symmetry, making them achiral.
- **Conclusion**: This complex does not exhibit optical isomerism.

- **Complex 2: [Co(en)2Cl2]**
- Here, "en" represents ethylenediamine, a bidentate ligand.
- The arrangement of ligands can lead to a chiral configuration, which lacks a plane of symmetry.
- **Conclusion**: This complex can exhibit optical isomerism.

- **Complex 3: [Co(en)3]**
- Similar to the previous complex, this one has three bidentate ligands.
- The arrangement can also lead to a chiral configuration, lacking a plane of symmetry.
- **Conclusion**: This complex can exhibit optical isomerism.

- **Complex 4: [Co(NH3)2Cl2]**
- This complex can also form a chiral arrangement depending on the spatial arrangement of ligands.
- **Conclusion**: This complex can exhibit optical isomerism.

3. **Final Conclusion**:
- Among the given complexes, only **Complex 1: [Co(NH3)3Cl3]** is not expected to exhibit optical isomerism due to the presence of a plane of symmetry in both its geometrical isomers.

### Answer:
**The complex that is not expected to exhibit optical isomerism is [Co(NH3)3Cl3].**