a) `text(Mono-or unidentate ligands)` : They have one donor atom, i.e. they supply only one electron pair to central metal atom or ion. Examples `F^-`, `Cl^-`, `Br^-`, `H_2O`, `NH_3`, `CN^-` , `NO_2^-`, `OH^-`, `CO`, `Py`, `H^-`

b) `text(Bidentate ligands)` : Ligands which have two donor atoms and have the ability to link with central metal ion at two positions are called bidentate ligands. Examples are : As shown in fig.1.

c) `text(Tridentate ligands)` : The ligands having three donor atoms are called tridentate ligands. Examples are: As shown in fig.2.

d) `text(Tetradentate ligands)` : These ligands possess four donor atoms. Examples are: As shown in fig.3.

e) `text(Pentadentate ligands)` : They have five donor atoms. For example, ethylenediamine triacetate ion(shown in fig.4).

f) `text(Hexadentate ligands)` : They have six donor atoms. The most important example is ethylenediaminetetraacetate ion(shown in fig.5).

On the basis of charge ligand can be classified in three category as follows :

a) `text(Anionic ligand)` : Anionic ligands are those ligands which have negative charge on them. See fig.1.

b) `text(Cationic ligand)` : Cationic ligands are those ligands which have positive charge on them. See fig.2.

c) `text(Neutral ligand)` : Neutral ligands have no charge on them. See fig.3.

(A) Ligands having one (or more) lone pair (or pairs) of electrons

a) Ligands which contain vacant `pi`-type orbitals that can receive back donated `pi`-electron from metal ion in low oxidation state.

e.g.: `CO`, `NO`, `CN^-`, `NC^-` , `R-Nunderset(rightarrow)(=)C`, `R_3P`, `R_3As`, `alpha,alpha-text(dipyridyl)`, `o-text(phenanthroline)`

b) All these ligands also have filled donor orbital in addition to vacant `pi`-type acceptor orbitals.

c) Thus in these complexes both metal and ligand function as donors and acceptors

(`M overset(sigma) underset(pi)rightleftarrow L`)

d) Ligands which do not have vacant orbitals to receive back donated electron from metals. e.g. `H_2O`, `NH_3`, `F^-`.

(B) Ligands having no lone pairs of electrons but have `pi` bonding electron

e.g. Ethylene, benzene, cyclopentadienyl-ion, `K[PtCl_3(C_2H_4)] text(Zisses salt)`

(a) `text(Polydentate ligands)` : have Flexidentate 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.

(b) `text(Ambidentate Ligand)` : Ligands which can ligate through two different atoms present in it are called ambidentate ligands. Examples of such ligands are the `NO_2^-` and `SCN^-` ions. `NO_2^-` 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. Such possibilities give rise to linkage isomerism in coordination compounds. 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^-`.

(c) `text(Chelating ligands)` : 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. 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.