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Ligand - Structure And Bonding, Chelating Agents, Metal-ligand Bonds In Biological Chemistry - Other uses

ligands ion atoms molecules

In inorganic chemistry, ligands are molecules or electrically charged atoms (ions) which are bonded to metal atoms or ions. The ligand changes the metal's ability to dissolve in or react with its surroundings. In biochemistry, ligands are defined as molecules, usually Figure 1. Two ligands (ammonia) each donate a pair of electrons to bond with a silver ion. (N = nitrogen, H = hydrogen, Ag = silver.) Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 2. Geometric symmetry of ligands around metal. Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 3. Geometric isomers have the same formulas but different symmetry and different properties. Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 4. Two orbitals, A and B, have the same energy in the isolated atom. Ligands (L) approach closer to A than to B, hence the energy of A rises and B falls. Illustration by Hans & Cassidy. Courtesy of Gale Group.


protein, that change the biological activity of other molecules by bonding with them. The inorganic meaning is more common, and will be the subject of this article.


Inorganic salts, which contain metal ions, do not dissolve in organic solvents such as benzene. However, by surrounding the metal ion with a chelate called a "crown ether" the desired solution can be made. The mining industry uses cyanide ions to dissolve gold out of the quartz rocks in which it is often found. The cyanide ligands are removed in subsequent chemical steps.


Resources

Books

Benarde, Melvin A. The Chemicals We Eat. New York: American Heritage Press, 1971.

Newton, David E. Consumer Chemistry Projects for Young Scientists. New York: Franklin Watts, 1991.

Periodicals

Schmidt, Karen F. "Mirror-Image Molecules." Science News 143 (May 29, 1993): 348-350.

Other

"Molecular Architecture." Unit 9 of The World of Chemistry. Videotape Series. University of Maryland at College Park. Gilbert Castellan, Nava Ben-Zvi, and Isidore Adler, project co-directors. The Annenberg/CPB Project, 1990.


Sara G. B. Fishman

KEY TERMS

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Coordination compound

—A molecule consisting of a metal atom and the ligands to which it is bound.

Covalent bond

—A chemical bond formed when two atoms share a pair of electrons with each other.

Ion

—An atom or molecule which has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).

Isomers

—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.

Orbital

—A region of space around an atomic nucleus likely to be occupied by an electron.

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