Which of the following is the most stable complex?
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When comparing complex stability, consider these factors in order:
1. \textbf{Chelate Effect:} Complexes with polydentate (chelating) ligands are significantly more stable than those with similar monodentate ligands.
2. \textbf{Ligand Strength:} Strong field ligands (like CN\(^-\), CO) generally form more stable complexes than weak field ligands (like H\(_2\)O, halogens).
3. \textbf{Metal Ion Charge/Size:} Higher charge and smaller size of the central metal ion generally lead to greater stability.
Step 1: Understanding Complex Stability.
The stability of a coordination complex is described by its stability constant (K) or formation constant (\(\beta\)). A higher value of the stability constant indicates a more stable complex. Several factors influence stability, including the nature of the metal ion, the nature of the ligand, and the chelate effect. Step 2: Analyzing the Ligands and the Chelate Effect.
(A) [Fe(CO)\(_5\)]: The ligand is Carbonyl (CO), a strong field ligand that forms strong \(\sigma\) and \(\pi\) bonds (synergic bonding). The complex is stable, but the oxidation state of Fe is 0.
(B) [Fe(CN)\(_6\)]\(^{4-}\): The ligand is Cyanide (CN\(^-\)), a very strong field ligand. The oxidation state of Fe is +2. Cyanide complexes are known to have very high stability constants.
(C) [Fe(C\(_2\)O\(_4\))\(_3\)]\(^{3-}\): The ligand is Oxalate (C\(_2\)O\(_4\)\(^{2-}\)), which is a bidentate ligand. Bidentate or polydentate ligands form rings with the central metal ion, a phenomenon known as the chelate effect. Chelation significantly increases the stability of a complex compared to complexes with analogous monodentate ligands. The oxidation state of Fe is +3.
(D) [Fe(H\(_2\)O)\(_6\)]\(^{3+}\): The ligand is Aqua (H\(_2\)O), which is a weak field ligand. The oxidation state of Fe is +3. Aqua complexes are generally less stable compared to those with strong field ligands or chelating ligands.
Step 3: Comparing the Stability.
We need to compare the stability of these complexes.
Comparing (C) and (D): The oxalate complex (C) is a chelate and will be much more stable than the aqua complex (D) due to the chelate effect.
Comparing (B) and (C): We are comparing a complex with a very strong monodentate ligand (CN\(^-\)) and a complex with a chelating ligand (C\(_2\)O\(_4\)\(^{2-}\)). While the chelate effect is powerful, the cyanide ligand is exceptionally strong and forms extremely stable complexes. The stability constant for [Fe(CN)\(_6\)]\(^{4-}\) is extremely large (log \(\beta\) \(\approx\) 35). The stability constant for [Fe(C\(_2\)O\(_4\))\(_3\)]\(^{3-}\) is also high due to chelation (log \(\beta\) \(\approx\) 20), but significantly lower than that of the cyanide complex.
Comparing (A) and (B): Metal carbonyls are stable due to synergic bonding. However, hexacyanoferrate(II) is renowned for its exceptional thermodynamic stability in aqueous solution.
Overall, the complex [Fe(CN)\(_6\)]\(^{4-}\) is known to be one of the most stable iron complexes due to the strong ligand field and high charge density of the CN\(^-\) ligand. Step 4: Final Answer.
The most stable complex among the given options is [Fe(CN)\(_6\)]\(^{4-}\).
