Which of the following is not optically active?

Options

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

Answer

To determine which of the given complexes is not optically active, we need to analyze each option for the presence of symmetry elements, such as a plane of symmetry or a center of symmetry. A complex is considered optically inactive if it has such symmetry elements, as these prevent the molecule from being chiral.

### Step-by-Step Solution:

1. **Identify the complexes:**
We have four complexes to analyze:
- Option A: [Co(en)₃]³⁺
- Option B: [Cr(ox)₃]³⁻
- Option C: cis-[CoCl₂(en)₂]
- Option D: trans-[CoCl₂(en)₂]

2. **Analyze Option A: [Co(en)₃]³⁺**
- Draw the structure: Cobalt is the central atom, surrounded by three ethylenediamine (en) ligands.
- Check for symmetry: The arrangement is asymmetrical, and there is no plane of symmetry.
- Conclusion: This complex is optically active.

3. **Analyze Option B: [Cr(ox)₃]³⁻**
- Draw the structure: Chromium is the central atom, surrounded by three oxalate (ox) ligands.
- Check for symmetry: The arrangement is also asymmetrical, and there is no plane of symmetry.
- Conclusion: This complex is optically active.

4. **Analyze Option C: cis-[CoCl₂(en)₂]**
- Draw the structure: Cobalt is the central atom, with two chloride (Cl) ligands and two ethylenediamine (en) ligands in a cis configuration.
- Check for symmetry: The cis arrangement does not have a plane of symmetry.
- Conclusion: This complex is optically active.

5. **Analyze Option D: trans-[CoCl₂(en)₂]**
- Draw the structure: Cobalt is the central atom, with two chloride (Cl) ligands and two ethylenediamine (en) ligands in a trans configuration.
- Check for symmetry: The trans arrangement has a plane of symmetry, as the two Cl ligands are opposite each other.
- Conclusion: This complex is optically inactive.

### Final Answer:
The complex that is not optically active is **Option D: trans-[CoCl₂(en)₂]**.