Problem 89 - A Complete Resource Book in Chemistry for JEE Main

Question

The stability constants of the complexes formed by a metal ions \(\left(\mathrm{M}^{2^{+}}\right)\)with \(\mathrm{NH}_{3}, \mathrm{CN}^{-}, \mathrm{H}_{2} \mathrm{O}\) and 'en' are of the order of \(10^{11}, 10^{27}, 10^{15}\) and \(10^{8}\) respectively. Then (en = ethylene diamine) (a) en is the strongest ligand (b) these values cannot predict the strength of the ligand (c) \(\mathrm{CN}^{-}\)is the strongest ligand (d) all ligands are equally strong

Step-by-Step Solution

Answer

(c) \( \mathrm{CN}^{-} \) is the strongest ligand.

Step 1: Understand Stability Constants

The stability constant (also known as the formation constant) is a measure of the affinity between a metal ion and a ligand. Larger values indicate a stronger interaction or more stable complex.

Step 2: Identify Given Stability Constants

We are given the stability constants for the complexes with the metal ion \( \mathrm{M}^{2+} \): \( 10^{11} \) for \( \mathrm{NH}_{3} \), \( 10^{27} \) for \( \mathrm{CN}^{-} \), \( 10^{15} \) for \( \mathrm{H}_{2} \mathrm{O} \), and \( 10^{8} \) for 'en' (ethylene diamine).

Step 3: Compare Stability Constants

Compare the given stability constants to determine which is the largest. This indicates the strongest ligand, as it forms the most stable complex with \( \mathrm{M}^{2+} \).

Step 4: Determine the Strongest Ligand

The largest stability constant is \( 10^{27} \), corresponding to \( \mathrm{CN}^{-} \). This means \( \mathrm{CN}^{-} \) is the strongest ligand among the given options.

Step 5: Select the Answer

Based on the comparison, \( \mathrm{CN}^{-} \) is the strongest ligand, making option (c) the correct choice.

Key Concepts

complex formation

In the world of chemistry, complex formation is a process where metal ions bond with molecules or ions called ligands. These ligands can be either neutral molecules, such as ammonia (9 NH3), or negatively charged ions, like cyanide (10 CN0). The formed structure is known as a complex, which consists of a central metal ion surrounded by these ligands. The arrangement of ligands around a metal ion is not random. Instead, the metal and the ligands interact in specific ways that maximize the stability of the complex. This stability is often measured by a number known as the stability constant or formation constant. Understanding complex formation is crucial because it influences the physical and chemical properties of substances, such as solubility and reactivity. Complexes play vital roles in various fields, including catalysis, medicine, and environmental science.

metal-ligand affinity

The affinity between a metal ion and a ligand is essentially a way to describe how well they bind to each other. Strong affinity means that the metal and ligand form a stable bond, leading to a stable complex. The stability constant is the numerical measure of this affinity: the higher the constant, the greater the affinity. Metal-ligand affinity depends on several factors, including the size and charge of the metal ion, the type of ligand, and the solvent in which the complex formation takes place. For instance, in an aqueous solution, metal ions with high positive charge and small size tend to have a higher affinity for polar ligands because of their ability to form strong electrostatic interactions. This concept is critical in predicting how different complexes will behave under various conditions, making it a fundamental factor in the field of coordination chemistry.

ligand strength

Ligand strength refers to the ability of a ligand to form a stable complex with a metal ion. It influences how strongly a ligand can bind to a metal ion. The stability constant gives a quantifiable measure of ligand strength: higher values indicate stronger ligands. In our example, we see different ligands with varying stability constants, ranging from ethylene diamine (2 en), which has the smallest constant, to cyanide (10 CN0), with the largest. This indicates that 10 CN0 is the strongest ligand among those considered, as it forms the most stable complex with the metal ion 12 M2+. Understanding ligand strength helps chemists in various fields, from designing more effective catalysts to creating metal complexes used in therapy or industrial applications. Observations of ligand behavior can lead to advancements in chemical processes and materials science.