A chelate is a type of coordination compound in which a single metallic ion is attached by coordinate covalent bonds to a molecule or an ion called a ligand. The term chelate comes from the Greek word chela, meaning "crab's claw." The term clearly describes the appearance of many kinds of chelates, in which the ligand surrounds the central atom in a way that can be compared to the grasping of food by a crab's claw.
Bonding in a chelate occurs because the ligand has at least two pairs of unshared electrons. These unshared pairs of electrons are regions of negative electrical charge to which are attracted cations such as the copper(I) and copper(II), silver, nickel, platinum, and aluminum ions. A ligand with only two pairs of unshared electrons is known as a bidentate ("two-toothed") ligand; one with three pairs of unshared electrons, a tridentate ("three-toothed") ligand, and so on.
The geometric shape of a chelate depends on the number of ligands involved. Those with bidentate ligands form linear molecules, those with four ligands form planar or tetrahedral molecules, and those with six ligands form octahedral molecules.
One of the most familiar examples of a chelate is hemoglobin, the molecule that transports oxygen through the blood. The "working part" of a hemoglobin molecule is heme, a complex molecule at whose core is an iron(II) ion bonded to four nitrogen atoms with coordinate covalent bonds.
Among the most common applications of chelates is in water softening and treatment of poisoning. In the former instance, a compound such as sodium tripolyphosphate is added to water. That compound forms chelates with calcium and magnesium ions, ions responsible for the hardness in water. Because of their ability to "tie up" metal ions in chelates, compounds like sodium tripolyphosphate are sometimes referred to as sequestering agents.
A typical sequestering agent used to treat poison victims is ethylenediaminetetraacetic acid, commonly known as EDTA. Suppose that a person has swallowed a significant amount of lead and begins to display the symptoms of lead poisoning. Giving the person EDTA allows that molecule to form chelates with lead ions, removing that toxic material from the bloodstream.