Question

(a) What is meant by the term chelate effect? (b) What thermodynamic factor is generally responsible for the chelate effect? (c) Why are polydentate ligands often called sequestering agents?

Short answer

(a) The chelate effect is the increased stability of a complex formed by a polydentate ligand when compared to a complex formed by monodentate ligands, due to the formation of a ring structure. (b) The thermodynamic factor responsible for the chelate effect is entropy, as the reaction with polydentate ligands produces fewer particles, leading to an increase in entropy. (c) Polydentate ligands are called sequestering agents because they form stable ring structures with metal ions, effectively trapping and controlling their availability in various systems.

Step-by-step solution

(a) Defining the Chelate Effect

The chelate effect refers to the increased stability of a complex formed by a polydentate (multi-dentate) ligand when compared to the stability of a complex formed by an equivalent combination of monodentate (single-dentate) ligands. A chelate complex is formed when a polydentate ligand coordinates with a central metal ion and forms a ring structure.

(b) Thermodynamic Factor Responsible for the Chelate Effect

The chelate effect is generally attributed to entropy. When a polydentate ligand forms a complex with a metal ion, fewer particles are produced compared to the reaction with monodentate ligands. The increase in entropy due to the release of a larger number of displaced monodentate ions or molecules is mainly responsible for the higher stability of the chelate complexes.

(c) Polydentate Ligands as Sequestering Agents

Polydentate ligands are often called sequestering agents because they can form stable ring structures with a central metal ion, effectively "sequestering" or trapping the metal ion in a chelate complex. This strong binding makes it difficult for the metal ion to participate in any undesired reactions, hence controlling its availability in systems such as industrial processes, biochemistry, and environmental chemistry.