Which of the following has an optical isomer?

Options

A. `[Co(en)(NH_3)_2]^(2+)`
B. `[Co(H_2O)_4(en)]^(3+)`
C. `[Co(en)_2(NH_3)_2]^(3+)`
D. `[Co(NH_3)_3Cl]^+`

Answer

To determine which of the given compounds has an optical isomer, we need to understand the concept of optical isomerism. Optical isomers, or enantiomers, are compounds that are non-superimposable mirror images of each other. This typically occurs in chiral molecules, which have at least one carbon atom bonded to four different substituents.

### Step-by-Step Solution:

1. **Identify the Compounds**: We need to analyze the given options to identify which one might have an optical isomer. The options given are:
- Option 1: [COENNH3]
- Option 2: [C(EN)(H2O)3]
- Option 3: [T(EN)2(NH3)2]
- Option 4: [CO(NH3)3CN]

2. **Analyze Option 1**: The structure COENNH3 does not appear to have a chiral center as it does not have four different substituents attached to a carbon atom. Therefore, it does not have optical isomers.

**Hint**: Look for a carbon atom with four different groups attached to determine chirality.

3. **Analyze Option 2**: The structure C(EN)(H2O)3 also does not have a chiral center. The presence of three identical water molecules (H2O) means that it cannot form non-superimposable mirror images.

**Hint**: Identify identical groups attached to the central atom; identical groups indicate no chirality.

4. **Analyze Option 3**: The structure T(EN)2(NH3)2 has a central atom that is bonded to two different EN groups and two NH3 groups. If the EN groups are different, this can create a chiral center, allowing for the formation of optical isomers.

**Hint**: Check if the groups attached to the central atom are different; this can lead to chirality.

5. **Analyze Option 4**: The structure CO(NH3)3CN does not have a chiral center either, as it has three identical NH3 groups and does not meet the criteria for optical isomerism.

**Hint**: Again, look for identical groups; they prevent the formation of chiral centers.

6. **Conclusion**: Based on the analysis, Option 3 (T(EN)2(NH3)2) is the only compound that has a chiral center and can form optical isomers.

### Final Answer:
**Option 3 has an optical isomer.**