A coordination compound is composed of one or more complex structural units, each of which has a central metal atom bound directly to a surrounding set of nonmetallic atoms or groups of atoms, called ligands. The class includes a number of important biological materials, such as vitamin B12, chlorophyll, and hemoglobin, the red coloring matter of blood. It also includes a number of industrially important materials used as dyestuffs and pigments; as agents for extracting, purifying, and analyzing metals; and as catalysts for preparing such useful organic substances as the polyethylene plastics. Principal types include aqua complexes, halide complexes, metal carbonyls, and metal nitrosyls. The metal atom in a coordination compound may be an electrically neutral atom or an ion. The ligands, which may also be neutral or charged, forms a chemical bond with the metal atom by sharing a pair of electrons with it. A ligand can attach to the atom by one bond (unidentate) or several bonds (multidentate). The latter configuration is cyclic, or aromatic, because it contains a ring of atoms. Multidentate compounds, called chelates, comprise an important group of compounds. Several coordination compounds have distinct geometric structures. Two common forms are the square planar, in which four ligands are arranged at the corners of a hypothetical square around the central metal atom, and octahedral, in which six ligands are arranged, four in a plane and one each above and below the plane. Altering the position of the ligands relative to one another can produce different compounds with identical chemical formulae. Thus, a cobalt ion linked to two chloride ions and four molecules of ammonia can occur in both violet and green forms according to the placement of the six ligands. Replacing a ligand can also affect the color. For example, a cobalt ion linked to six ammonia molecules is yellow, but replacing one of the ammonia molecules with water turns it rose red, while replacing all six ammonia molecules with water molecules turns it purple. Some synthetic pigments and dyes are coordination compounds. Several natural coordination compounds are responsible for color in biological systems. For example, heme, the red pigment of blood, is an iron coordination compound, and chlorophyll, the green plant pigment, is based on a magnesium coordination compound; both are multidentate. The iron atom in heme is joined to four nitrogen atoms, which are part of a heterocyclic structure called a porphyrin. Chlorophyllís magnesium ion is similarly bound to nitrogen porphyrin atoms. In one class of organometallic coordination compounds, two molecules of an unsaturated cyclic hydrocarbon, which lacks one or more hydrogen atoms, bond on either side of a metal atom to form a "sandwich." The result is a highly stable aromatic system. These compounds have provided impetus to the development of organometallic chemistry. Multidentate ligands, useful in some industrial processes as chelating agents, can sequester a metal by forming a coordination compound with it thereby changing its properties. Thus, such compounds can be used, for example in softening hard water, in extracting metals from ores, and in medicine to remove poisonous concentration of metal ions from the body. Metal carbonyls are neutral compounds in which several molecules of carbon monoxide are coordinated to a metal atom. Important in some metallurgical processes, metal carbonyls are generally toxic and often volatile. Their volatility, which assists in the purification of metals, also facilitates their deposition as thin, even films. In carbon monoxide poisoning, a carbon monoxide molecule attaches to the iron in heme, which adversely affects it ability to transport oxygen. A principal application of coordination compounds is their use as catalysts, which serves to alter the rate of chemical reactions. Certain complex metal catalysts, for example, play an essential part in the manufacture of polyethylene and polypropylene. In the future, coordination compounds as industrial catalysts could possibly be used to convert carbon monoxide into useful petroleum products.