Any member of a class of substances containing at least one metal-to-carbon bond in which the carbon is part of an organic group. Most organometallic compounds are solids, although some are liquids and others are gases. While some organometallic compounds are stable, those containing electropositive elements, such as lithium, sodium, or aluminum, are spontaneously flammable and highly toxic. Organometallic compounds may form covalent bonds, in which electrons are shared equally between two atoms; multicentre covalent bonds, in which an electron pair is shared between more than two atoms; and ionic bonds, in which the electron pair is donated by only one atom. Polar organometallic compounds (one end of the compound is more negative than the other end) are formed as a result of covalent bonding in which there exists an unequal sharing of electrons between a metal and carbon atom. Polar organometallic compounds are valuable in the synthesis of certain materials and chemicals. For example, alkylaluminum is reacted with titanium salts to catalyze the polymerization of unsaturated hydrocarbons. This reaction is commonly used to catalyze the polymerization of ethylene to polyethylene, a type of plastic. Organometallic compounds containing tin are used as pharmaceuticals, pesticides, and fire retardants. Examples of well-characterized organometallic compounds include tetracarbonylnickel, Ni(CO), a volatile nickel compound used in the purification of nickel, and ferrocene, Fe(CH), a remarkably stable compound in which an iron atom is sandwiched between two hydrocarbon rings.
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