(i) Polydentate ligands whose structures permit the attachment of two or more donor sites to the same metal ion simultaneously, thus closing one or more rings are called chelating ligands and the compounds formed are known as chelate compounds.

(ii) A chelate may be defined as a ring structure formed by the combination of a polydentate ligand having two or more donor atoms with a metal ion forming part of the ring.

(iii) The process of formation of chelates is called chelation.

(iv) Chelate complexes are more stable than ordinary complexes in which the ligand is a monodentate.

(v) This increased stability of the compound due to chelation is called the chelate effect.

(vi) Generally the chelate complexes with 5 or 6 membered rings are more stable.

(vii) Out of these, 5 membered rings are very stable when they involve saturated ligands.

(viii) On the other hand 6-membered ring structures acquire maximum stability when they involve unsaturated ligands containing conjugate double bond. This is due to the resonance effects involving metal `d`-orbitals and ligand `p`-orbital electrons.