ligand, charged or uncharged molecule with one or more unshared pairs of electrons that can attach to a central metallic atom or ion to form an aggregate known as a complex ion (see chemical bond). Some ligands that share electrons with metals form very stable complexes. Some common bases that act as ligands are water and ammonia molecules and halide, hydroxide, acetate, cyanide, thiocyanate, and nitrite anions. These ligands are monofunctional, i.e., they are attached by one unshared pair of electrons during complexing. Polyfunctional ligands, which bind to the metal ion with two or more pairs of electrons, are called chelates (see chelating agents). Ethylenediamine-tetraacetate, a commonly used chelating compound, has six pairs of electrons to bind to metal ions. Electron-donating functional groups containing nitrogen, oxygen, sulfur, phosphorous, or carbon may act as ligands in complex biological systems. For example, in enzymes that need complexed metal ions to function, mercapto (sulfur-containing) groups and amino (nitrogen-containing) groups act as chelating agents; these groups fix the metal ion in a specific position. Other biologically important molecules, such as chlorophyll, vitamin B₁₂, and heme, also have nitrogen-containing groups that donate electrons and have a chelating function.

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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