Nomenclature:
Organometallic compounds are normally named as substituted metals, e.g. alkyl metal or alkyl metal halide.
Organomagnesium compounds are generally referred to as Grignard reagents.
Examples:  CH₃Li = methyl lithium, CH₃MgBr = methyl magnesium bromide.

Physical Properties:

• Organometallic are usually kept in solution in organic solvents due to their very high reactivity (especially with H₂O, O₂ etc.)

• Organosodium and organopotassium compounds are essentially ionic compounds.
• Organolithiums and organomagnesiums have a s bond between a C atom an the metal.
• These are very polar, covalent bonds due to the electropositive character of the metals.

Reactivity:

methyl chloride	methyl lithium	methyl magnesium bromide
CH3Cl	CH3Li	CH3MgBr
The images show the electrostatic potentials for methyl chloride, methyl lithium and methyl magnesium bromide.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. In the alkyl halide, the methyl group has lower electron density (blue), and is an electrophile.  In methyl lithium, the methyl group has higher electron density (red) and is a nucleophile. In methyl magnesium bromide, the methyl group is less electron rich that methyl lithium. Therefore, organometallic compounds react as electron rich or anionic carbon atoms i.e. as carbanions, which means they will function as either bases or nucleophiles. It is reasonable to think of these organometallic compounds as R- M+ Can you think of other types of anions where this "dual" reactivity is also very important ?

Basicity:

The following equation represents the loss of a proton from a generic hydrocarbon forming a carbanion:

• Organolithium and organomagnesium compounds are strong bases since the negative charge is on carbon.
• Simple carbanions are strong bases, (see pKa's below) since the C is not very electronegative (compared to N or O)
• In the presence of weak acids, RLi and RMgX protonate giving the hydrocarbon.

• Implications: RLi or RMgX CANNOT be used in the presence of acidic hydrogens such as -OH, -NH or -SH units. What happens if you do ? 

Compound Structure  pKa  2-methylpropane 71 ethane 62 methane 60 ethene 45 benzene 43 ammonia 36 ethyne 25 ethanol 16 water 15.7

The table shows the pKa's of a selection of representative systems. Note that the hydrocarbons are very weak acids, implying that the carbanions will be strong bases. Question Explain the pKa order: ethane > ethene > ethyne

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