Which of the following is not optically active?

Options

A. `[Co(en)_(3)]^(3+)`
B. `[Cr(O x)_(3)]^(3-)`
C. cis-`[CoCl_(2)(en)_(2)]^(+)`
D. trans-`[CoCl_(2)(en)_(2)]^(+)`

Explanation

To determine which of the given coordination compounds is not optically active, we will analyze each option based on the concept of optical activity, which is determined by the ability of a compound to have non-superimposable mirror images.

### Step-by-Step Solution:

1. **Understanding Optical Activity**:
- A compound is optically active if it has a chiral center, meaning it cannot be superimposed on its mirror image. If the mirror image can be superimposed, the compound is optically inactive.

2. **Analyzing Each Compound**:
- We will analyze the given compounds one by one.

3. **Compound A: [Co(en)3]**:
- This compound has a central cobalt atom surrounded by three bidentate ethylenediamine (en) ligands.
- Drawing the mirror image shows that the two structures cannot be superimposed due to the arrangement of the en ligands.
- **Conclusion**: This compound is optically active.

4. **Compound B: [Cr(ox)3]**:
- Here, chromium is the central atom with three oxalate (ox) ligands, which are also bidentate.
- The mirror image of this compound also results in a non-superimposable structure.
- **Conclusion**: This compound is optically active.

5. **Compound C: [CoCl2(en)2] (cis form)**:
- This compound has a cobalt atom with two chloro ligands and two bidentate en ligands in a cis arrangement.
- The mirror image will also be non-superimposable.
- **Conclusion**: This compound is optically active.

6. **Compound D: [CoCl2(en)2] (trans form)**:
- In this case, the cobalt atom has two chloro ligands positioned opposite each other and two en ligands on the sides.
- When we draw the mirror image, we find that it can be superimposed on the original structure.
- **Conclusion**: This compound is optically inactive.

7. **Final Answer**:
- The compound that is not optically active is **D: [CoCl2(en)2] (trans form)**.

### Summary:
The answer to the question is **D**. The trans form of [CoCl2(en)2] is not optically active because its mirror image can be superimposed on itself.