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 complex compounds in coordination chemistry often depends on several factors, such as the nature and configuration of the ligands, ligand exchange kinetics, and the specific metal center involved. The stability of the compounds can be compared based on the concept of ligand field strength and chelating effects, where chelate ligands create more stable complexes.

The given options are:

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

Let's analyze the most stable compound:

Formal Charge and Chelation: The compound \([\text{CoCl}_2(\text{en})_2] \text{NO}_3\) involves ethylenediamine (en), a bidentate ligand that forms a chelate ring with the central metal atom. Chelate ligands enhance stability due to the chelation effect; they form more stable rings with the central atom rather than monodentate ligands.

Ligand Field Strength and Stability: Ethylenediamine is a stronger field ligand than water, ammonia, or chloride ion. Thus, it can increase the ligand field stabilization energy, contributing further to the stability of the complex.

Based on these considerations, the compound \([\text{CoCl}_2(\text{en})_2] \text{NO}_3\) is expected to be the most stable choice among the given options due to the presence of chelating en ligands.

Answer 2

What Does Stability Mean?
In a complex compound, stability means how strong the bond is between the central metal and the ligands (molecules attached to the metal). Stronger bonds make the complex more stable. 
Two things help stability: 
- Strong Ligands: Some ligands hold the metal tightly (like NH₃ or NO₂⁻), while others are weaker (like Cl⁻ or H₂O). 
- Chelation: If a ligand grabs the metal at two points (like ethylenediamine, or "en"), it makes the complex extra stable.

Let’s Look at the Options
Option 1: [Co(NH₃)₆]SO₄
- Metal: Cobalt (Co³⁺). - Ligands: 6 NH₃ (ammonia, a strong ligand). - Stability: Pretty good because NH₃ is strong, but all ligands are single-point (no chelation). 
Option 2: [Co(NH₃)₄(H₂O)Br]NO₃
- Metal: Co³⁺. - Ligands: 4 NH₃ (strong), 1 H₂O (weak), 1 Br⁻ (weak). - Stability: Less stable than Option 1 because H₂O and Br⁻ are weak ligands. - Option 3: [Co(NH₃)₅(NO₂)]NO₃
- Metal: Co³⁺. - Ligands: 5 NH₃ (strong), 1 NO₂⁻ (strong). - Stability: Very good because all ligands are strong, but still no chelation. 
Option 4: [CoCl(en)₂]NO₃
- Metal: Co³⁺. - Ligands: 1 Cl⁻ (weak), 2 en (ethylenediamine, a strong ligand that grabs at two points). - Stability: The best! Even though Cl⁻ is weak, "en" is a chelating ligand, making the complex super stable.

Which One Wins?
Complexes with chelating ligands (like "en" in Option 4) are the most stable because they hold the metal tightly at two points. So, Option 4: [CoCl(en)₂]NO₃ is the most stable.