COORDINATION COMPOUND

any of a class of substances with chemical structures in which a central metal atom is surrounded by nonmetallic atoms or groups of atoms, called ligands, joined to it by chemical bonds. Coordination compounds include such substances as vitamin B12, hemoglobin, and chlorophyll; dyestuffs and pigments; and catalysts used in preparing organic substances. The metal atom in a coordination compound may be an electrically neutral atom or an ion. The ligands may also be neutral or charged. A ligand 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 are called chelates and comprise an important group of compounds. Many 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 the same chemical formula. Thus, a cobalt ion linked to two chloride ions and four molecules of ammonia can occur in both green and violet forms according to how the six ligands are placed. Replacing a ligand also can affect the colour. A cobalt ion linked to six ammonia molecules is yellow. Replacing one of the ammonia molecules with a water molecule turns it rose red. Replacing all six ammonia molecules with water molecules turns it purple. Some synthetic dyes and pigments are coordination compounds. Moreover, a number of natural coordination compounds are responsible for colour in biological systems. Heme, the red pigment of blood, is an iron coordination compound, for example. The plant pigment chlorophyll is based on a magnesium coordination compound. Both heme and chlorophyll are multidentate. In heme the iron atom is joined to four nitrogen atoms, which are part of a heterocyclic organic structure called a porphyrin. The magnesium ion in chlorophyll is similarly bound to nitrogen porphyrin atoms. A very stable class of organometallic coordination compounds has provided impetus to the development of organometallic chemistry. In these compounds two molecules of an unsaturated cyclic hydrocarbon, which lacks one or more hydrogen atoms, bond on either side of a metal atom, forming a sandwich. This results in a highly stable aromatic system. Multidentate ligands are useful in some industrial processes as chelating agents. They can sequester a metal ion by forming a coordination compound with it. This changes its properties in such a way that it can be used, for example, in softening hard water or extracting metals from ores. In medicine, it can remove poisonous concentrations of metal ions from the body. Metal carbonylsneutral compounds in which several molecules of carbon monoxide are coordinated to a metal atomare also important in some metallurgical processes. These compounds are often volatile. Their volatility assists in the purification of metals and also facilitates their deposition as thin, even films. Metal carbonyls are generally toxic. In carbon monoxide poisoning a carbon monoxide molecule attaches to the iron in heme, which affects its ability to transport oxygen. A major application of coordination compounds is their use as catalysts, which serve to alter the rate of chemical reactions. Certain complex metal catalysts, for example, play a key role in the production of polyethylene and polypropylene. A future use of coordination compounds as industrial catalysts could be to convert carbon monoxide into useful petroleum products.