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

Options

A. `[Co(en) (NH_(3))_(2) Cl_(2)]^(+)`
B. `[Co(en)_(3)]^(3+)`
C. `[Co(en)_(2) Cl_(2)]^(+)`
D. `[Co(NH_(3))_(3)Cl_(3)]`

Explanation

To determine which of the following complex species is not expected to exhibit optical isomerism, we need to analyze each option based on the principles of coordination chemistry, particularly focusing on the geometry of the complexes and the presence of chiral centers.

### Step-by-Step Solution:

1. **Understanding Optical Isomerism**:
- Optical isomerism occurs when a compound can exist in two forms that are mirror images of each other, known as enantiomers. For a complex to exhibit optical isomerism, it must lack an internal plane of symmetry.

2. **Analyzing Each Complex**:
- **Option 1: [Co(en)(NH3)2Cl2]+**
- This complex has a bidentate ligand (en) and two monodentate ligands (NH3 and Cl). The geometry is likely octahedral (MAA2B2 type). The cis form can exhibit optical isomerism since it can form enantiomers.

- **Option 2: [Co(en)3]3+**
- This complex consists solely of the bidentate ligand (en). It is also octahedral (MA3 type) and can exhibit optical isomerism as it can form two enantiomers due to its chiral nature.

- **Option 3: [Co(en)2Cl2]+**
- This complex has two bidentate ligands (en) and two monodentate ligands (Cl). It can also exist in different geometrical forms (cis and trans) and can exhibit optical isomerism due to the presence of chiral centers.

- **Option 4: [Co(NH3)3Cl3]**
- This complex consists of three monodentate ligands (NH3) and three monodentate ligands (Cl). It can exist in facial (fac) and meridional (mer) isomers. However, both forms have an internal plane of symmetry and do not exhibit optical isomerism.

3. **Conclusion**:
- The complex that does not exhibit optical isomerism is **Option 4: [Co(NH3)3Cl3]**.

### Final Answer:
The complex species that is not expected to exhibit optical isomerism is **Option 4: [Co(NH3)3Cl3]**.