Amongst the following , the most stable complex is :
(a) `[Fe(H_(2)O)_(6)]^(3+)` (b) `[Fe(NH_(3))_(6)]^(3+)` (c) `[Fe(C_(2)O_(4))_(3)]^(3-)` (d) `[FeCl_(6)]^(3-)` .

Options

A. `[Fe(H_(2)O)_(6)]^(3+)`
B. `[Fe(NH_(3))_(6)]^(3+)`
C. `[Fe(C_(2)O_(4))_(3)]^(3-)`
D. `[FeCl_(6)]^(3-)`

Answer

To determine the most stable complex from the given options, we will analyze the stability of each complex based on their ligands and structures.

### Step 1: Identify the Coordination Number and Geometry
All the complexes have a coordination number of six, indicating that they all adopt an octahedral geometry.

### Step 2: Analyze Each Complex
1. **Complex (a):** `[Fe(H₂O)₆]³⁺`
- Ligand: Water (H₂O)
- This complex is formed by six water molecules coordinated to iron in the +3 oxidation state.

2. **Complex (b):** `[Fe(NH₃)₆]³⁺`
- Ligand: Ammonia (NH₃)
- Similar to the first complex, this one has six ammonia molecules coordinated to iron in the +3 oxidation state.

3. **Complex (c):** `[Fe(C₂O₄)₃]³⁻`
- Ligand: Oxalate (C₂O₄²⁻)
- This complex consists of three oxalate ligands. Oxalate is a bidentate ligand, meaning each oxalate can form two bonds with the metal center, leading to the formation of a chelate ring.

4. **Complex (d):** `[FeCl₆]³⁻`
- Ligand: Chloride (Cl⁻)
- This complex contains six chloride ions coordinated to iron in the +3 oxidation state.

### Step 3: Stability Considerations
- **Chelate Effect:** The stability of coordination complexes can be significantly affected by the nature of the ligands. Chelating ligands (like oxalate) form stable ring structures with the metal ion, which increases the overall stability of the complex.
- **Non-chelating Ligands:** Water, ammonia, and chloride are monodentate ligands, which do not form such stable ring structures.

### Step 4: Conclusion
Among the complexes analyzed:
- Complex (c) `[Fe(C₂O₄)₃]³⁻` is the most stable due to the presence of chelating oxalate ligands that form stable five-membered rings with the iron ion. The chelate effect provides additional stability compared to the other complexes, which do not form such rings.

### Final Answer
The most stable complex is **(c) `[Fe(C₂O₄)₃]³⁻`**.

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