Organometallic Compounds

Chapter 14: 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 = methyl lithium, CH3MgBr = methyl magnesium bromide.

Physical Properties:

Structure:
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 magnesium bromide
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 ? Hydroxide, HO- and alkoxides RO-

Basicity:

The following equation represents the loss of a proton from a generic hydrocarbon forming a carbanion: hydrocarbons are weak acids (high pKa) 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 
% s character of the hydridization of the orbital accepting the -ve charge