Question

What is meant by the chelate effect? Give an example.

Answer 1

Hint: Chelate is a term related to coordination compounds. When a di- or polydentate ligand uses its two or more donor atoms together to bind a single metal ion then it is said to be a chelate ligand. The number
 of such ligating groups is called denticity of the ligand.

Complete answer: When a ligand is bound to a metal ion through one single donor atom then it is called unidetate but when it uses two donor atoms it is said to be a bidentate ligand. $C{l}^{-}$ , $H_{2}O$are unidentate ligands and $C_2{O}_4^{-}$ , $H_{2}NC{H}_{2}C{H}_{2}N{H}_2$are bidentate ligands. Similarly ligands having more than two donor atoms are called polydentate ligands $N{\left(C{H}_{2}C{H}_{2}N{H}_2\right)}_3$is a polydentate ligand. When a di- or polydentate ligand uses its two or more donor atoms together to bind a single metal ion then it is said to be a chelate ligand. When a ligand uses its two or more donor atoms simultaneously a ring-like structure is formed. The formation of such ring-like structures is called a chelate effect. Compounds having chelate rings are more stable as compared to other compounds. An example of such a complex is ethylenediamine-cadmium. The structure of this complex is:

Note:
The coordination number of the metal complex is defined as the number of ligand donor atoms to which metal is directly bonded. The central atom/ion and the ligands attached to it are enclosed in a square bracket and the term collectively used is the coordination sphere.