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 (H 2 O), ammonia (NH 3 ), and carbon monoxide (CO) and the anion s cyanide (CN - ), chloride (Cl - ), and hydroxide (OH - ). Rarely, ligands are cation s and electron-pair acceptors ( electrophile s). Organic ligands include EDTA (see chelate ) and nitrilotriacetic acid. Biological systems rely on ligands such as the porphyrin in hemoglobin and chlorophyll , and numerous cofactor s 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.