Ligand

Some ligands can form more than one bond to a single metal atom. These are called chelating agents. The name comes from the Greek word chele, meaning "claw." The ligands surround the metal atom and hold it as if in a claw. Because they hold metals so strongly, chelates are also referred to as "metal scavengers." They effectively remove metal atoms and prevent them from reacting with anything else.

Figure 6. The porphyrin molecule surrounds the metal, bonding to it by nitrogen atoms at the four corners of a square. Other ligands may bond above and below the plane of the square. (N = nitrogen, C = carbon, H = hydrogen) Illustration by Hans & Cassidy. Courtesy of Gale Group.

One of the best-known chelating agents is ethylene-diaminetetraacetic acid, or EDTA. It contains 32 atoms, six of which can bond to a single metal atom.

EDTA is a common food preservative. Foods contain ions of iron, zinc, magnesium, and other metals. These are natural components of food substances, but they hasten the chemical reactions which cause flavor and color to deteriorate. EDTA added to foods forms strong, stable bonds to the metal ions, blocking their chemical activity. EDTA is also used to treat lead poisoning in human beings. The EDTA-lead complex is safely excreted in body waste.