Atom, group (&see; functional group), or molecule attached to a central atom, usually of a transition element, in a coordination or complex compound (&see; bonding). It is almost always the electron-pair donor (nucleophile) in a covalent bond. Common ligands include the neutral molecules water (H2O), ammonia (NH3), and carbon monoxide (CO) and the anions cyanide (CN−), chloride (Cl−), and hydroxide (OH−).
Rarely, ligands are cations and electron-pair acceptors (electrophiles). Organic ligands include EDTA (&see; chelate) and nitrilotriacetic acid. Biological systems rely on ligands such as the porphyrin in hemoglobin and chlorophyll, and numerous cofactors are ligands. In chelates, the ligand attaches at more than one point, sharing more than one electron pair, and is called bidentate or polydentate—having two or many “teeth.” The ligands in a complex may be the same or different.
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