/luy"geuhnd, lig"euhnd/, n.1. Biochem. a molecule, as an antibody, hormone, or drug, that binds to a receptor.2. Chem. a molecule, ion, or atom that is bonded to the central metal atom of a coordination compound. Cf. complexing agent.[1945-50; < L ligandus, ger. of ligare to bind, tie]
* * *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 polydentatehaving two or many "teeth." The ligands in a complex may be the same or different.
* * *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)6]2+, 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 monodentate ligand, or through two or more atoms, in which case it is called a didentate or polydentate ligand.
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