Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

Why is \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2^{+}}\), nearly \(10^{10}\) times more stable than \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+} ?\) (en \(=\) enthlene diamine \()\) (a) \(\mathrm{NH}_{3}\) is the weakest ligand (b) 'en' is a chelating ligand and forms thermody namically more stable complexes (c) six \(\mathrm{NH}_{3}\) ligands cause steric hindrance around the \(\mathrm{Ni}^{+}\)centre (d) \(\mathrm{NH}_{3}\) evaporates easily and causes instability to the \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) complex

Step-by-Step Solution

Answer

Option (b): 'en' is a chelating ligand and forms thermodynamically more stable complexes.

Step 1: Identify the Ligands in Each Complex

In the complex \([\mathrm{Ni}(' \mathrm{en}')_3]^{2+} \), the ligand 'en' stands for ethylenediamine, which is a bidentate ligand, meaning it can form two bonds with the metal ion. In the complex \([\mathrm{Ni}(\mathrm{NH}_3)_6]^{2+}\), ammonia, \(\mathrm{NH}_3\), is a monodentate ligand, meaning it can form only one bond with the metal ion.

Step 2: Understand the Concept of Chelation

Chelating ligands, like 'en', form a ring structure with the metal ion by coordinating with multiple bonds. This increases the stability of the complex due to the chelate effect, which is a phenomenon where complex formation with chelating ligands is favored over complex formation with the same number of monodentate ligands due to entropic considerations.

Step 3: Compare the Stability of the Complexes

The stability of a complex is influenced by the nature of the ligands. The chelate effect of the ethylenediamine makes \([\mathrm{Ni}(' \mathrm{en}')_3]^{2+}\) far more stable compared to \([\mathrm{Ni}(\mathrm{NH}_3)_6]^{2+}\). Chelation leads to more thermodynamically stable complexes, making \([\mathrm{Ni}(' \mathrm{en}')_3]^{2+}\) approximately \(10^{10}\) times more stable.

Step 4: Determine the Correct Answer

Based on the analysis, option \((b)\) that 'en' is a chelating ligand and forms thermodynamically more stable complexes is the correct reason why \([\mathrm{Ni}(' \mathrm{en}')_3]^{2+}\) is nearly \(10^{10}\) times more stable than \([\mathrm{Ni}(\mathrm{NH}_3)_6]^{2+}\).

Key Concepts

Chelation

Chelation is an essential concept in coordination chemistry. It refers to the process where a single ligand forms multiple bonds with a central metal ion. This creates a ring-like structure that is incredibly stable. Ethylenediamine, often abbreviated as 'en', is a classic example of a chelating ligand as it can form two bonds to the metal due to its two electron-donating nitrogen atoms. This bidentate nature allows 'en' to "wrap around" the metal ion, enhancing the stability of the entire complex. The key benefit of chelation is what chemists call the "chelate effect." The more points of attachment a ligand has, the more difficult it is for the ligand to detach, thus making the complex significantly more stable. As multiple bonds are formed between the ligand and the metal, this creates constraints that improve the thermodynamic stability. Compared to six separate ammonia ligands, which attach one by one, a chelating ligand like 'en' creates fewer opportunities for the ligand to be replaced or detached.

Ligands

Ligands are molecules or ions that donate pairs of electrons to a central metal ion in coordination compounds. They essentially "coordinate" with metals to stabilize the compound. The nature of ligands can vary widely, affecting the properties of the resulting metal complex substantially.

• Monodentate ligands: Ligands like ammonia (\(\mathrm{NH}_3\)) that coordinate through only one donor atom are called monodentate ligands. They form just one bond to the metal and are easier to replace compared to chelating ligands.
• Bidentate ligands: Ligands like ethylenediamine (\(\mathrm{en}\)) can form two simultaneous bonds with the metal center and form stable ring structures.
• Impact on stability: The type of ligand affects the stability of these complexes, as seen in the greater stability of complexes containing chelating ligands over those with monodentate ones.

Stability of Complexes

The stability of coordination complexes is largely influenced by the type of ligands and their interactions with the metal center. Complexes with chelating ligands are usually more stable than those with comparable non-chelating ligands, a phenomenon driven by factors like ligand denticity, entropy and enthalpy considerations.

• Chelate effect: As mentioned before, the stability provided by chelating ligands such as 'en' is due to the chelate effect. This effect strengthens the complex through increased entropy change during chelation, as the formation of ring structures leads to more stable and organized structures.
• Thermodynamic stability: Due to the greater number of bonds formed between the chelating ligands and the metal ion, the resulting complex's thermodynamic stability is markedly increased, as seen in the example where \([\mathrm{Ni}(' \mathrm{en}')_3]^{2+}\) is significantly more stable than \([\mathrm{Ni}(\mathrm{NH}_3)_6]^{2+}\).
• Configurational stability: Chelating ligands decrease the freedom of movement, severely limiting the rearrangements that the complex can undergo, leading to higher kinetic stability.

Understanding the factors influencing stability helps explain why certain metal-ligand combinations dominate in biological systems, industrial applications, and beyond.