The complex cation which has two isomers is :

Options

A. ` [Co(H_2O)_6] ^(3+)`
B. ` [Co(NH_3)_5Cl]^+`
C. ` [Co(NH_3)_5NO_2]^(2+ )`
D. ` [Co(NH_3)_5Cl]^(2+)`

Answer

To determine which complex cation has two isomers, we need to analyze the given options based on the types of isomerism they can exhibit.

### Step-by-Step Solution:

1. **Understanding Isomerism**:
- Isomerism in coordination compounds can occur in various forms, including structural isomerism (such as linkage isomerism) and stereoisomerism (such as cis-trans isomerism).

2. **Analyzing the Options**:
- **Option 1: Co(H2O)6^3+**:
- This complex has six water molecules as ligands. Since all ligands are the same, it cannot exhibit any isomerism (neither structural nor stereoisomerism).

- **Option 2: Co(NH3)5Cl^2+**:
- This complex has five ammonia molecules and one chloride ion. The arrangement of ligands does not allow for any isomerism since the ammonia ligands are identical and the chloride is fixed in one position.

- **Option 3: Co(NH3)5NO2^2+**:
- This complex has five ammonia ligands and one nitro (NO2) ligand. The nitro ligand is ambidentate, meaning it can bind through either the nitrogen (N) or the oxygen (O). This allows for the possibility of linkage isomerism, resulting in two different isomers.

- **Option 4: Co(NH3)5Cl^2+**:
- This option is identical to option 2 and will not exhibit isomerism for the same reasons.

3. **Conclusion**:
- The only complex cation that can exhibit two isomers due to the presence of an ambidentate ligand (NO2) is **Option 3: Co(NH3)5NO2^2+**.

### Final Answer:
The complex cation which has two isomers is **Co(NH3)5NO2^2+**.

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