Ligand Definition in Chemistry

A ligand is an atom, ion, or molecule that donates or shares one or more of its electrons through a covalent bond with a central atom or ion. It is a complexing group in coordination chemistry that stabilizes the central atom and determines its reactivity. Ligands are usually considered to be Lewis bases, although a few cases of Lewis acid ligands exist.

Some sources only consider ligands to be functional groups that bind to a central metal complex. In these cases, the bonds formed within the ligand may range from covalent to ionic in nature.

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Complex Ion Definition in Chemistry

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Ligand Examples

Monodentate ligands have one atom that can bind to a central atom or ion. Water (H₂O) and ammonia (NH₃) are examples of neutral monodentate ligands.

A polydentate ligand has more than one donor site. Bidentate ligands have two donor sites. Tridentate ligands have three binding sites. ​1,4,7-triazaheptane (diethylenetriamine) is an example of a tridentate ligand. Tetradentate ligands have four binding atoms. A complex with a polydentate ligand is called a chelate.

An ambidentate ligand is a monodentate ligand that can bind in two possible places. For example, The thiocyanate ion, SCN^-, can bind to the central metal at either the sulfur or the nitrogen.

Sources

• Cotton, Frank Albert; Geoffrey Wilkinson; Carlos A. Murillo (1999). Advanced Inorganic Chemistry. Wiley-Interscience. ISBN 978-0471199571.
• Jackson, W. Gregory; Josephine A. McKeon; Silvia Cortez (2004). "Alfred Werner's Inorganic Counterparts of Racemic and Mesomeric Tartaric Acid: A Milestone Revisited." Inorganic Chemistry. 43 (20): 6249–6254. doi:10.1021/ic040042e