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. 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 Werners Inorganic Counterparts of Racemic and Mesomeric Tartaric Acid: A Milestone Revisited. Inorganic Chemistry. 43 (20): 6249–6254. doi:10.1021/ic040042e