Which one of the following metal complexes is most stable?
(a) [Co(en)(NH3)4]Cl2
(b) [Co(en)3]Cl2
(c

Q: What exactly is the chelating effect?

The chelating effect refers to the enhanced thermodynamic stability of a complex containing a multidentate (chelating) ligand compared to a similar complex containing only monodentate ligands. This is primarily an entropy-driven phenomenon.

Q: Why is ethylenediamine (en) considered a bidentate ligand?

Ethylenediamine ($H_2N-CH_2-CH_2-NH_2$) has two nitrogen atoms, each possessing a lone pair of electrons. Both nitrogen atoms can simultaneously donate their electron pairs to the same central metal ion, forming a stable five-membered ring.

Q: Does the oxidation state of Cobalt affect the stability in this specific question?

While the oxidation state does affect stability generally, in this specific question, all the given options have Cobalt in the same $+2$ oxidation state (balanced by two $Cl^-$ counter ions). Therefore, the differentiating factor is solely the chelating effect.

Q: Can ammonia ($NH_3$) form chelate rings?

No, ammonia is a monodentate ligand. It only has one nitrogen atom with a lone pair, so it can only form a single bond with the metal and cannot loop back to form a ring.

Q: How do we calculate the number of chelate rings formed by a ligand?

The number of chelate rings formed by a single ligand molecule is equal to its denticity minus one. For a bidentate ligand like '$en$' (denticity = 2), it forms $2 - 1 = 1$ ring. For a hexadentate ligand like EDTA (denticity = 6), it forms $6 - 1 = 5$ rings.

Question

Which one of the following metal complexes is most stable?

Options:

(A) $[Co(en)(NH_3)_4]Cl_2$

(B) $[Co(en)_3]Cl_2$

(C) $[Co(en)_2(NH_3)_2]Cl_2$

(D) $[Co(NH_3)_6]Cl_2$

Sigma Percentile · Accepted Answer

### The Secret to Stability: The Chelating Effect

When dealing with coordination complexes, one of the most fascinating phenomena that dictates their thermodynamic stability is the **chelating effect**. Imagine a central metal ion as a VIP guest, and the ligands as bodyguards. If the bodyguards hold hands and form a ring around the VIP, the protection is significantly stronger than if they were just standing independently. This is exactly what multidentate ligands do!

### Analyzing the Ligands

In our problem, we are given four different cobalt complexes. To determine which one is the most stable, we must look closely at the ligands attached to the central $Co^{2+}$ ion. We have two types of ligands in play here:

1. **Ammonia ($NH_3$)**: This is a monodentate ligand. It has only one donor atom (the nitrogen) and can form only a single coordinate bond with the metal. It does not form any rings.
2. **Ethylenediamine ($en$)**: This is a bidentate ligand. It possesses two nitrogen atoms, each with a lone pair of electrons. When it binds to the central metal, it attaches at two adjacent sites, forming a stable five-membered ring. This ring is known as a **chelate ring**.

### Counting the Chelate Rings

The rule of thumb is simple: **the greater the number of chelate rings, the higher the thermodynamic stability of the complex**. This happens because replacing monodentate ligands with multidentate ones increases the total number of independent molecules in the solution, leading to a favorable increase in entropy ($\Delta S > 0$).

Let's evaluate our options:

*   **(d) $[Co(NH_3)_6]Cl_2$**: Contains six monodentate ammonia ligands. **Zero chelate rings**.
*   **(a) $[Co(en)(NH_3)_4]Cl_2$**: Contains one bidentate '$en$' ligand. **One chelate ring**.
*   **(c) $[Co(en)_2(NH_3)_2]Cl_2$**: Contains two bidentate '$en$' ligands. **Two chelate rings**.
*   **(b) $[Co(en)_3]Cl_2$**: Contains three bidentate '$en$' ligands. **Three chelate rings**.

### The Final Verdict

Since the complex $[Co(en)_3]Cl_2$ forms the maximum number of chelate rings (three), it completely envelops the central cobalt ion in a highly stable, cage-like structure. Therefore, it benefits the most from the chelating effect and stands as the most stable complex among the given choices.

Animated Solution for Chemistry - Coordination Compounds: Which one of the following metal complexes is most stable?

Select Answer:

Visualized Solution

$Analyzing the Options$

$The Chelating Effect$

$Evaluating Ligands$

$\text{Counting Rings: Options A & D}$

$Counting Rings: Option C$

$Counting Rings: Option B$

$Conclusion$

$The Way Forward$

The Sigma Insight: Bonding and Crystal field

The Secret to Stability

Analyzing the Ligands

Counting the Chelate Rings

The Final Verdict

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