organometallic compounds

Organometallic compounds are a very large class of substances in which the molecules contain at least one metal-to-carbon bond in which the carbon is part of an organic group. The class includes such compounds as the antiknock gasoline additive, tetraethyl lead, and the remarkably stable ferrocene, a compound in which an iron atom is "sandwiched" between two hydrocarbon rings. Organometallic compounds, which have played a major role in the development of the science of chemistry, are employed extensively as catalysts and intermediates in the laboratory and in industry. These compounds are typically termed as either main-group compoundsóbased on the s-block metals of group 1 and 2, and the heavier p-block elements (groups 13-15)óor transition metal compounds, based on the transition metals of the d- and f-block groups. The organometallic compounds exhibit great variation in their physical and chemical properties. Most are solids, particularly those with aromatic or ring-shaped hydrocarbon groups, but some are liquids and some are gases. Their heat and oxidization stability varies greatly. Although some are quite stable, several compounds of such electropositive elements as lithium, sodium, and aluminum are highly toxic, especially those that are volatile. The type of carbon-metal bonds involved in the organometallic compounds in the primary determinant of their properties. Some are ordinary covalent bonds, in which atoms share pairs of electrons. Others are multicenter covalent bonds, in which the bonding involves more than two atoms. In ionic bonds, a third type, the bonding electron pair is donated by only one atom. In donor-acceptor bonds, multiple bonds between carbon atoms connect the metal atom to hydrocarbons. This bond polarity imparts to many compounds reactivities that have made them important in chemical synthesis. The organometallic halides (Grignard reagents), for example, are employed extensively in synthetic organic chemistry, as are organolithium and organoboron compounds. Alkylaluminum compounds are also employed in organic synthesis. Used with titanium salts, they are important catalysts in the polymerization of such unsaturated hydrocarbons as ethylene and propylene. It is probable that the mechanism that the mechanism of action of the titanium-aluminum alkyl catalysts involves interaction between the titanium atoms and the double bonds of the hydrocarbons. Organometallic compound containing tin, mercury, and lead are all commercially important. Several organotin compounds are used as pesticides, pharmaceuticals, stabilizers for polyvinyl chloride, and fire retardants. Environmental pollution by the highly toxic compound methylmercury has led to stringent controls on the discharge of mercury from chemical plants into rivers, lakes, and oceans. The use of tetraethyl lead, a major industrial organolead compound used extensively for decades as an antiknock agent in gasoline, has been curtailed in recent years because, when it burns, it emits lead particulates into the atmosphere. Carbon monoxide reacts readily with many transition-metal atoms to form metal carbonyls, themselves a class of organometallics. One of the earliest to be discovered, the volatile nickel compound tetracarbonylnickel, became the basis of a process for purifying nickel. Metal carbonyls are used as catalysts in many reactions in the petrochemical industry.