Organometallics
An organometallic compound is an organic compound in which a metal atom is attached directly to a carbon, with the exclusion of metallic salts of organic acids. Organometallic compounds have been prepared using practically all the metals. The first known organometallic compound was diethylzinc Zn(C2H5)2. Other examples include Grignard compounds such as methyl magnesium iodide CH3MgI; metallic alkyls such as butyllithium C4H9Li, tetraethyl lead Pb(C2H5)4, triethyl aluminum (C2H 5)3Al, tetrabutyl titanate Ti(OC4H9)4, sodium methylate CH3ONa, copper pththalocyanine C32H16N8Cu; and metallocenes. Some organometallic compoun ds are highly toxic or flammable; others are coordination compounds (i.e., compounds formed between a metal ion and a nonmetallic ion or molecule, with the nonmetallic component referred to as a complexing agent). Many organometallic compounds are powerful catalysts.
The carbon-metal bonds in organometallic range in nature from essentially ionic to primarily covalent. The identity of the metal atom has much greater influence on the carbon-metal bond that the structure of the organic component. Carbon-sodium and carbon-potassium bonds are largely ionic; while carbon-lead, carbon-tin, carbon-thallium, and carbon-mercury bonds are mostly covalent. Carbon-lithium and carbon-magnesium bonds lie between the two extremes.
Organometallic bonds become more reactive as the percentage of ionic character of the metal-carbon bond increases. Thus, alkyl sodium and alkyl potassium compounds are highly reactive, and are very powerful bases. They react explosively with water, and burst into flame when exposed to air. Organomercury and organolead compounds are less reac tive, and remain stable in air. All organometallic compounds are poisonous. They are generally soluble in nonpolar solvents.
The metal-carbon bonds of organometallic compounds of lithium and magnesium have relatively large ionic characters, which make them strong bases and powerful nucleophiles.
Organomagnesium halides were discovered by the French chemist Victor Grignard in 1900; they are now known as Grignard reagents. They are usually prepared by reacting an organic halide with magnesium metal in an ether solvent. Grignard reagents are very strong bases, and so react with any compound that has a hydrogen atom attached to an electrone gative element such as oxygen, nitrogen, or sulfur. They react with water and alcohols in what are essentially acid-base reactions, as if they contained a carbanion; they will also remove protons that are much less acidic than those of water and alcohols. They carry out nucleophilic attack at saturated carbons when they react with ethylene oxide epoxy groups (oxiranes). A particularly important reaction undergone by Grignard reagents occurs with the unsaturated carbons of carbonyl groups. In the case of aldehydes and ketones containing carbonyl groups, the highly nucleophilic Grignard reagent contributes its electron pair to form a bond with the carbon atom. One electron pair of the carb onyl group shifts to the oxygen atom. This step results in the formation of an alkoxide ion that is subsequently protonated, leading to the formation of an alcohol and a compound MgX2 (where X is a halogen). The reaction of Grignard reagents with carbonyl compounds is used to prepare primary, secondary, and tertiary alcohols.
Organolithium reagents, which like Grignard reagents are very strong bases, react with compounds containing carbonyl groups in essentially the same way to produce alcohols. These reagents are often prepared by reducing an organic halide with lithium metal in an ether solvent. As an example, butyl bromide can be reacted with lithium metal in diethyl ether to create a solution of the metallic alkyl butyllithium.
This is the complete article, containing 552 words
(approx. 2 pages at 300 words per page).
