SPECIAL LIGANDS:

(1) Polydentate ligands : have Flexi dentate character, it is not necessary that all the donor atoms present in the Polydentate ligands should form coordinate bonds with central metal atom or ion.

(2) Ambidentate Ligands: Ligands which can ligate through two different atoms present in it are called Ambidentate ligands. Examples of such ligands are the NO₂, and SCN ions. NO₂ , ion can coordinate through either the nitrogen or the oxygen atoms to a central metal atom/ion. Similarly, SCN¯ ion can coordinate through the sulphur or nitrogen atom. There are certain ligands which have two or more donor atoms but during formation of complexes only one donor atom is attached to metal ion. Such ligands are called Ambidentate ligands such as CN¯, CNS¯

(3) Chelating ligands:

(1) 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.

(2) 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.

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

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

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

(6) In the complex ion given below, 5membered rings are formed. So all these are called chelate complexes

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

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

(11) 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.

IllUSTRATIVE EXAMPLE (1): Match the Column A and B

	Column A		Column  B
(1)	Ligand contains one donor site	(a)	hexadented
(2)	Ligand contains two donor site	(b)	tridented
(3)	Ligand contain three donor site	( c)	tetradented
(4)	Ligand contain four donor site	(d)	bidented
(5)	Ligand contain six donor site	(e)	unidented

SOLUTION:  (1) - (e); (2) - (d); (3) - (b); (4) - (c); (5) - (a)