Question

The most stable complex among the following is:

• A. \([ \text{Ni(NH}_3)_6 ]\text{Cl}_2\)
• B. \([ \text{Ag(NH}_3)_2 ]\text{Cl}\)
• C. \([ \text{Fe(en)}_2\text{Cl}_2 ]^+\)
• D. \( \text{K}_4[ \text{Fe(CN)}_6 ] \)

Hint:

Strong field ligands like CN\(^{-}\) and chelating ligands enhance the stability of complexes. Among monodentate ligands, CN\(^{-}\) gives maximum stability due to strong metal-ligand bonding.

Answer 1

The Correct Option is D

Stability of complexes depends on factors like: - Nature of ligand (strong or weak field) - Chelate effect - Charge on central metal ion - Nature of metal Analysis: - (A) \([ \text{Ni(NH}_3)_6 ]\text{Cl}_2\): Involves monodentate ligand (NH\(_3\)) with moderate field strength. Stability is moderate.
- (B) \([ \text{Ag(NH}_3)_2 ]\text{Cl}\): Contains only two ammonia ligands; silver(I) forms labile complexes. Low stability.
- (C) \([ \text{Fe(en)}_2\text{Cl}_2 ]^+\): Contains bidentate ethylenediamine ligands (en) which provide some chelation. However, presence of Cl\(^{-}\) weakens stability.
- (D) \( \text{K}_4[ \text{Fe(CN)}_6 ] \): Cyanide (CN\(^{-}\)) is a strong field ligand forming very stable low-spin complexes with Fe(II). This makes it highly stable.
Therefore, the most stable complex is: \[ \boxed{K_4[Fe(CN)_6]} \]