Organometallic Compounds

The s-metals form a range of organometallic compounds which are used in inorganic and organic synthesis, mainly as sources of carbanionsie. C containers.

In Group 1, organometallics have been made with all of the elements. The alkyllithium compounds are the most useful.

Organolithium compounds

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.

Li4Me4:tetrahedral 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.

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.

Reagent Product
Nucleophilic reaction R2CO/H2O R3COH
Nucleophilic reaction RCHO/H2O R2CHOH
Double displacement EXn ERn
Double displacement SO2Cl2 R2SO2
Double displacement SOCl2 R2SO
Oxidation O2 ROH
Protolysis HX RH
Protolysis MeOH RH
β-hydrogen elimination M-CH2-CH2-R M-R + CH2=CH2

Other Organometallics

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.