Question

Which complex compound is most stable?

• A. \([\text{Co(NH}_3\text{)}_6]_2(\text{SO}_4)_3\)
• B. \([\text{Co(NH}_3\text{)}_4 (\text{H}_2\text{O)Br}] (\text{NO}_3)_2\)
• C. \([\text{Co(NH}_3\text{)}_3 (\text{NO}_3)_3]\)
• D. \([\text{CoCl}_2(\text{en})_2] \text{NO}_3\)

Answer 1

The Correct Option is D

The stability of a complex compound depends on factors such as the nature of the ligands, the oxidation state of the central metal ion, and the geometry of the complex. Let's analyze the given options to determine which complex compound is the most stable.

1. \([\text{Co(NH}_3\text{)}_6]_2(\text{SO}_4)_3\): In this compound, ammonia (NH₃) acts as a ligand, which is a neutral and strong-field ligand. However, there is no chelation involved, which can lower stability compared to other compounds.
2. \([\text{Co(NH}_3\text{)}_4 (\text{H}_2\text{O)Br}] (\text{NO}_3)_2\): This compound includes both NH₃ and H₂O as ligands. Water is a weaker ligand than NH₃, and the presence of a mono-dentate ligand like H₂O makes the complex less stable than complexes with only strong-field ligands.
3. \([\text{Co(NH}_3\text{)}_3 (\text{NO}_3)_3]\): This compound contains NO₃^-, which is not a strong-field ligand, and there is no chelation, making it less stable compared to complexes with chelating ligands.
4. \([\text{CoCl}_2(\text{en})_2] \text{NO}_3\): Here, ethylenediamine (en) is a bidentate ligand, meaning it can form two bonds with the central metal ion, resulting in a chelate complex. Chelation generally increases the stability of the complex. This compound has both chelating ligands and a favorable geometry, making it very stable.

Based on the analysis above, the compound \([\text{CoCl}_2(\text{en})_2] \text{NO}_3\) is the most stable due to the presence of chelating ligands (ethylenediamine). Chelating ligands form more stable rings with the central metal ion, thereby enhancing the stability of the complex.