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The ions or molecules which surround a metal ion or metal atom are called ligands. They  are attached to central metal atom or central metal ion by co-ordinate bond. They donate  at least one electron pair to central metal atom or ion. The atom of ligand which donates the electron pair to central metal atom or ion is called donor atom.

Classification  :

This classification is based upon the number of donor atoms present in ligand. They are classified as :

1. Mono-dentate ligands: Those having only one donor atom are called mono-dentate ligands or they form one co-ordinate bond with metal ion.

Example: In H2O donor atom is O.

In NH3 donor atom is N.

In CO donor atom is O.

2.Bi-dentate ligands: Those having two donor atoms are called bi-dentate ligands. They form   co-ordinate bond with central metal atom or ion at two places.



3. Tridentate ligands:

Those  having three donor atoms are called tri-dentate ligands. They form  co-ordinate bond with central metal atom at three places.

Example:  1) ( C15H11N3) terpyridine


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2)       tris (pyrazolyl ) hydro-borates.

Similarly, there are tetradentate ligand and hexadentate ligands.

Example of tetradentate ligand:

1) C6H9NO6 – Nitrilo tri acetic acid

2) (NH2CH2CH2)3N –  Tris (2-amino ethyl) amine

Example of Hexadentate ligand:

Ethylene diamine tetra acetic acid (EDTA)


In EDTA, two nitrogen and four oxygen atoms are donor atoms. So it is hexadentate.

Naming of ligands:

The ligands may be negative , neutral or positive.

1) The negative  ligands whose names end with ‘ide’ have this suffix replaced by  ‘O’.

Ex.  Cl (chloro) ,  CN (cyano), Br (bromo) , O (oxo), OH (hydroxo)

2)  The negative ligands whose names end with ‘ite’ or ‘ate’ become ‘ito ‘ & ‘ato’ respectively .

CO3– – (carbonato), CH3COO (acetato), SCN (thiocyanato), SO4 (sulphato), C2O4 (oxalato) , NO2 (nitrito) .

3 ) The name of neutral  molecule is written as CO (carbonyl) , H2N CH2.CH2NH( ethylenediamine ) , H2O (aqua), NH3 (amine).

4) The name of positive ligand   ends with ‘ium’.

NH2NH3+ (hydrazinium), NO2+ (nitronium), NO+ (nitrosonium).

According to the number of ligands- prefix mono, di, tri etc are used before the name of ligand. They are written in alphabetical order. If di ,tri tetra ,penta , hexa etc are present in the name of ligand , then bis , tris , tetrakis, pentakis,hexakis is used  before their name  respectively.