organometallic chemistry, the reactions and use of a class of compounds (R-M) that contain a covalent bond between carbon and metal. They are prepared either by direct reaction of the metal with an organic compound or by replacement of a metal from another organometallic substance. Their use is based on the polar R-M bond, in which the carbon atom carries a partial negative charge, and on the nature of the metal atom. In synthesis they act as nucleophiles that can bond with relatively positive carbon atoms in compounds such as alkyl halides, aldehydes, and ketones. For example, the Grignard reagent, RMgX (where X equals Br, Cl, or I), and organolithium compounds react with ketones to give secondary alcohols. In industry, butyllithium is used for the polymerization of isoprene in the manufacture of synthetic rubber; metalloorganic compounds serve as catalysts. The semimetals, boron, and silicon are important organometallics; organoboranes are used in synthesis, while organosilicones are polymerized to manufacture plastics and elastomers. Many organometallics are toxic primarily because of the toxicity of the metal. For example tetraethyl lead has been banned as gasoline additive and the conversion of mercury to mercury alkyls by fish has had serious consequences in Japan.