The s-metals form a range of organometallic compounds which are used in inorganic and organic synthesis, mainly as sources of carbanions, ie. C– containers.
In Group 1, organometallics have been made with all of the elements. The alkyllithium compounds are the most useful.
These are found in ether or hydrocarbon solutions, and are formed from the reaction of the metal and the alkylbromide.
Methyllithium exists as the tetramer, Li4(CH3)4, which is tetrahedral in structure.
In Group 2, the organometallic compounds range from covalent in character with beryllium and magnesium to ionic further down the group. The organoberyllium and organomagnesium compounds tend to be four coordinate in structure.
The most frequently used organomagnesium compounds in synthesis are the alkylmagnesium bromide compounds, and these are known as Grignard reagents.
These are prepared by dissolving magnesium metal in solution of the alkylbromide in ether.
The Grignard reagent, EtMgBr, has tetrahedral coordination in the ether solution.
In solution, equilibria exist between the grignard reagent and the dialkylmagnesium and the magnesium dibromide compounds. This is known as the Schlenk equilibrium.
Other organometallics are formed from Grignard reagents in a transmetallation process, where one metal atom takes the place of another, in a standard inorganic reaction.
Reactions of Organometallics
These are used as a source of carbanions, C–, and used widely in organic synthesis.
The principal use is the increase in the number of carbon atoms in a compound, by a reaction involving nucleophilic addition. Other reactions include oxidation, protolysis (addition of H to an alkyl group), β-hydrogen elimination, and displacement reactions. These reactions are summarized in the table.
|β-hydrogen elimination||M-CH2-CH2-R||M-R + CH2=CH2|
In these reactions, more elaborate compounds are formed. The cyclopentadiene group is labeled Cp. The Cp– ligand is a six electron delocalized species, and hence is aromatic like benzene.
The compound Cp2Fe is known as ferrocene.