Return to Contents Chapter 14: Organometallic Compounds Ch 14 contents

Organometallic Compounds

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 = methyllithium, CH3MgBr = methylmagnesium bromide.

Physical Properties:

Structure: Look at the electronegativities of the metals Li, Na, K and Mg compared to C and the other atoms we have seen so far (e.g. N, O, F, Cl etc).  In C-L and C-Mg which atom is more electronegative ? What about C-O, C-N or C-Cl bonds ?
 
H
2.1
  He
Li
1.0
Be
1.5
  B
2.0
C
2.5
N
3.0
O
3.5
F
4.0
Ne
Na
0.9
Mg
1.2
 

Al
1.5

Si
1.8
P
2.1
S
2.5
Cl
3.0
Ar
K
0.8
Ca
1.0
Sc Ti  V  Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br
2.8
Kr
Partial Periodic Table with Pauling Electronegativities

Reactivity:

Electrostatic potential for methyl chloride
Electrostatic potential of methyl lithium
Electrostatic potential of methyl magnesium bromide
methyl chloride
methyllithium
methylmagnesium 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+
QUESTION Can you think of other types of anions where this "dual" reactivity is also very important ? ANSWER

Basicity:

The following equation represents the loss of a proton from a generic hydrocarbon forming a carbanion: hydrocarbons are weak acids (high pK<sub>a</sub>) STRONG bases !

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  ANSWER


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Dr. Ian Hunt, Department of Chemistry, University of Calgary