Which of the following is the most stable complex ?

Options

A. `[Fe(CO)_(5)]`
B. `[Fe(H_(2)O)_(6)]^(3+)`
C. `[Fe(C_(2)O_(4))_(3)]^(3-)`
D. `[Fe(CN)_(6)]^(3-)`

Explanation

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

### Step-by-Step Solution:

1. **Identify the Complexes**: The complexes given are:
- **Complex A**: Fe(PO)5
- **Complex B**: Fe(H2O)6^3+
- **Complex C**: Fe(C2O4)3^3-
- **Complex D**: Fe(CN)6^3-

2. **Analyze the Ligands**:
- **Complex A (Fe(PO)5)**: The ligand PO (phosphine oxide) is a monodentate ligand, meaning it can attach to the metal at only one site.
- **Complex B (Fe(H2O)6^3+)**: Water (H2O) is also a monodentate ligand. This complex has six water molecules coordinated to the iron.
- **Complex C (Fe(C2O4)3^3-)**: The oxalate ion (C2O4^2-) is a bidentate ligand, meaning it can attach to the metal at two sites. This allows for the formation of chelate rings, which enhances stability.
- **Complex D (Fe(CN)6^3-)**: The cyanide ion (CN^-) is a monodentate ligand. However, it is known to form very stable complexes due to the strong field strength of the CN^- ligand.

3. **Evaluate Stability**:
- **Chelation Effect**: Complex C (Fe(C2O4)3^3-) benefits from the chelation effect, where the bidentate oxalate ligand forms rings with the metal ion, leading to increased stability.
- **Field Strength**: Complex D (Fe(CN)6^3-) has strong field ligands (CN^-), which also contributes to its stability, but it does not have the chelation effect that Complex C has.

4. **Conclusion**: Among the complexes analyzed, Complex C (Fe(C2O4)3^3-) is the most stable due to the chelation effect provided by the bidentate oxalate ligands.

### Final Answer:
The most stable complex is **Fe(C2O4)3^3-**.

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