Question

Which of the following is the most stable complex?

• A. [Fe(CO)\(_5\)]
• B. [Fe(CN)\(_6\)]\(^{4-}\)
• C. [Fe(C\(_2\)O\(_4\))\(_3\)]\(^{3-}\)
• D. [Fe(H\(_2\)O)\(_6\)]\(^{3+}\)

Hint:

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.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The stability of a coordination complex depends on various factors including the nature of the central metal ion, the field strength of the ligands, and prominently, the chelate effect.
Step 2: Key Formula or Approach:
Identify the denticity of the ligands involved. Complexes with chelating ligands (bidentate or polydentate) exhibit exceptional thermodynamic stability.
Step 3: Detailed Explanation:
1. The ligands $H_{2}O$ and $CO$ are strictly unidentate (monodentate). The $CN^{-}$ ion is also unidentate.

2. The $C_{2}O_{4}^{2-}$ (oxalate) ion is a bidentate ligand, meaning it binds to the metal center at two points simultaneously.

3. When a bidentate ligand binds, it forms a ring structure with the central metal. This phenomenon is called the chelate effect. The formation of three stable 5-membered rings in $[Fe(C_{2}O_{4})_{3}]^{3-}$ causes a highly positive entropy change ($\Delta S>0$), making the formation thermodynamically very favorable and providing immense stability compared to similar complexes with only unidentate ligands.
Step 4: Final Answer:
$[Fe(C_{2}O_{4})_{3}]^{3-}$ is the most stable complex due to the chelate effect.