LIGAND

in chemistry, any atom or molecule attached to a central atom, usually a metallic element, in a coordination or complex compound. The atoms and molecules used as ligands are almost always those that are capable of functioning as the electron-pair donor in the electron-pair bond (a coordinate covalent bond) formed with the metal atom. Examples of common ligands are the neutral molecules water (H2O), ammonia (NH3), and carbon monoxide (CO) and the anions cyanide (CN-), chloride (Cl-), and hydroxide (OH-). Occasionally, ligands can be cations (e.g., NO+, N2H5+) and electron-pair acceptors. The ligands in a given complex may be identical, as the CO ligands in Fe(CO)5 and the H2O ligands in Ni(H2O)2/6+, or different, as the CO and NO ligands in Co(CO)3(NO). Attachment of the ligand to the metal may be through a single atom, in which case it is called a unidentate ligand, or through two or more atoms, in which case it is called a bidentate or polydentate ligand.