What are the different types of ligands?

Ligands are ions or molecules that form coordination compounds or coordination complexes by binding to the central metal atom or ion. There are 9 types of ligands based on their denticity (the number of binding sites with the central metal atom)  

1. Monodentate ligands bind the metal atom or ion at only one place. They are also known as one-toothed ligands. Examples - Bromide ion, fluoride ion, ammonia, water, carbon monoxide
2. Bidentate ligands bind with the same central metal atom or ion through two different donor atoms. They are also known as chelating ligands. Examples - Acetylacetonate ion, Oxalate ion, Phenanthroline, Dimethylglyoxime 
3. Tridentate ligands bind three lone pairs of electrons to the central metal atom or ion. Examples - Diethylenetriamine
4. Tetradentate ligands bind four lone pairs of electrons to the central metal atom or ion. Examples - Triethylenetetramine, Nitrilotriacetato
5. Pentadentate ligands, which have five donor atoms, are rare because of ring strain. Example - Ethylenediaminetriacetato.
6. Hexadentate ligands have six donor atoms. Example - Ethylenediaminetetraacetate
7. Ambidentate ligands have more than one potential donor atom but only one of them at a time can bond with the central metal atom. Example - Thiocyanate ion (NCS–) which can bind to the central metal atom or ion with either nitrogen or sulfur atoms. NO2–  and SCN–.
8. Bridging ligands have lone pairs of donor atoms that bind to two or more metal atoms. Examples- Hydroxide, Chloride, Oxide, Carbonyl
9. Cyclic or ring ligands have cyclic structures with several lone pairs of atoms available for donation. Chelates that are formed with metal atoms are more stable than non-cyclic complexes. Examples – Vitamin B12, Chlorophyll, Hemoglobin