Which of the following is most stable complex ?

Options

A. `[Co(NH_(3))_(6)]^(+2)`
B. `[Co(NH_(3))_(6)]^(+3)`
C. `[Co(en)_(2)(NH_(3))_(2)]^(+3)`
D. `[Co(en)_(3)]^(+3)`

Answer

To determine which of the given coordination complexes is the most stable, we will analyze the complexes based on the ligands involved and their properties.

### Step-by-Step Solution:

1. **Identify the Coordination Complexes:**
- Option 1: [Co(NH3)6]²⁺
- Option 2: [Co(NH3)6]³⁺
- Option 3: [Co(en)2(NH3)2]³⁺
- Option 4: [Co(en)3]³⁺

2. **Understand the Ligands:**
- In options 1 and 2, the ligand is ammonia (NH3), which is a monodentate ligand. This means it can donate only one pair of electrons to the central metal atom (Co).
- In options 3 and 4, the ligand is ethylenediamine (en), which is a bidentate ligand. This means it can donate two pairs of electrons to the central metal atom.

3. **Assess Chelation:**
- Chelating ligands, like ethylenediamine, form more stable complexes because they can wrap around the metal ion, creating a ring structure. This increases the stability of the complex.
- The more bidentate ligands present, the more stable the complex will be due to the formation of multiple bonds with the metal ion.

4. **Analyze Each Option:**
- **Option 1:** [Co(NH3)6]²⁺: Contains 6 monodentate NH3 ligands.
- **Option 2:** [Co(NH3)6]³⁺: Similar to option 1 but with a higher charge, which can increase stability slightly due to stronger electrostatic interactions.
- **Option 3:** [Co(en)2(NH3)2]³⁺: Contains 2 bidentate en ligands and 2 monodentate NH3 ligands. This provides some stability due to the presence of bidentate ligands.
- **Option 4:** [Co(en)3]³⁺: Contains 3 bidentate en ligands, which maximizes the chelation effect and provides the highest stability among the options.

5. **Conclusion:**
- Based on the analysis, **Option 4: [Co(en)3]³⁺** is the most stable complex due to the presence of three bidentate ethylenediamine ligands, which form a stable chelate structure.

### Final Answer:
**The most stable complex is Option 4: [Co(en)3]³⁺.**