a. Mono or unidentate ligands: The ligand molecule or ion which has only one donor atom or one point of attachment and can coordinate with the metal ion at only one site in a complex is called unidentate or monodentate ligand.
eg. Cl, OH–, NH3, H2O, etc. (Explanation + Example)
b. Poly or multidentate ligands: The ligand molecule or ion which has two or more donor atoms or points of attachments and can be linked to the same metal in a complex using two or more donating sites is called poly or multidentate ligands. Thus, multiple sites of ligands are used in the coordination with metal. Based on the number of donor atoms, polydentate ligands are further classified as bidentate, tridentate, tetradentate, etc.
1. Bidentate ligands have two donor atoms.
eg. Ethylenediammine (en)
2. Tridentate ligands have three donor atoms.
eg. Diethylenetriammine (dien)
3. Tetradentate ligands have four donor atoms.
eg. Triethylenetetraammine (trien)
4. Hexadentate ligands have six donor atoms.
eg. Ethylenediamminetetraacetate (EDTA)
(Explanation + Examples) [1]
c. Ambidentate ligands: Ambidentate ligands are the ligands which have two or more donor atoms capable of forming coordinate bonds; however only one donor atom is utilized during complex formation.
eg. 2 NO group can form complexes by utilizing either N or O as donor atom, but not both. This results in formation of either MNO2 or MONO complex respectively.