Which of the following does not show optical isomerism ?

Options

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

Explanation

To determine which of the given coordination compounds does not show optical isomerism, we need to analyze the symmetry of each compound. Optical isomerism occurs when a compound can exist in two non-superimposable mirror images, which typically requires the absence of a plane of symmetry.

### Step-by-Step Solution:

1. **Understanding Optical Isomerism**:
- Optical isomerism arises in chiral molecules, which lack any symmetry elements such as a plane of symmetry, center of symmetry, or axis of symmetry.

2. **Analyzing the Options**:
- We need to evaluate each compound provided in the options to check for the presence of a plane of symmetry.

3. **Option 1: [Co(NH3)3Cl3]**:
- This compound can exist in two geometrical forms: facial (fac) and meridional (mer).
- Both forms have a plane of symmetry. For example, in the meridional isomer, if you slice through the center, the left and right halves are identical.
- **Conclusion**: This compound does not show optical isomerism.

4. **Option 2: [Ni(en)(Cl)2]**:
- Here, "en" stands for ethylenediamine, which is a bidentate ligand.
- The compound can exist in a cis form where the two Cl ligands are adjacent. This arrangement does not have a plane of symmetry.
- **Conclusion**: This compound shows optical isomerism.

5. **Option 3: [Co(en)3]**:
- This compound is also chiral as it lacks any plane of symmetry due to the arrangement of the bidentate ligands.
- **Conclusion**: This compound shows optical isomerism.

6. **Option 4: [Cr(NH3)4Cl2]+**:
- Similar to option 2, this compound can also exist in a cis form where the two Cl ligands are adjacent.
- This arrangement does not have a plane of symmetry.
- **Conclusion**: This compound shows optical isomerism.

### Final Answer:
The compound that does not show optical isomerism is **Option 1: [Co(NH3)3Cl3]**.