_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The first three periodic groups
Table 3.1. Electronegativity and reactivity of alkyls
Pauling
electro-
Metal negativity Alkenes Ethers RC N R 2C=O C0 2 RCOCl H 2 0 Oz
Ca 1.00 + ±a + + + + + +
b
Mg 1.31 + + + + + +
b
Be 1.57 + + + + + +
b
Zn 1.65 ±c + + +
Cd 1.69 + +
Hg 2.00
Na 0.93 + + + + + + + +
b
Mg 1.31 + + + + + +
b
Al 1.61 +
+ +e + + + +
Si 1.90
Li 0.98 ± ±a + + + + + +
b
Be 1.57
+ + + + +
B 2.04 +
•Rates vary greatly. hDoes not cleave ethers; forms adducts. cR 2 Zn does not react, RZnCI
does. dDoes not react by addition, reduction is the main reaction. ecomplicated by
reduction and enolization.
polarity as indicated by Pauling electronegativity values (Allred-Rochow electronegativities
do not tit very well). Similarly, reactivity decreases in passing from left
to right across a given Period e.g. Li> Be > B and Na > Mg >Al> Si. These
trends are summarized in Table 3.1. which shows the qualitative relationship
between electronegativity (and hence M-C bond polarity) and the ability to
react with some common functional groups.
The organic compounds ofthe elements ofGroup 1 to III possess vacant valence
orbitals and hence are able to function as Lewis acids. They therefore form
adducts with electron pair donors such as ethers, tertiary amines and phosphines.
This coordination chemistry is discussed further on p. 82. In general the lower the
electronegativity of the central atom, the stronger the Lewis acidity. If the Lewis
base also has an active (acidic) hydrogen atom, the initially formed adduct may
eliminate hydrocarbon e.g.
Me 2 Zn + MeOH ------> ~(MeZnOMe), + CH 4
Me 2 Zn + Ph 2 NH ------> Î(MeZnNPh 2 ) 2 + CH 4
The products, if monomeric, would still possess donor and acceptor sites; thus
MeZnOMe monomer would ha ve two oxygen !o ne pairs and two acceptor orbi tais
on zinc. Association therefore occurs to form oligomers or, in some cases,
polymers. The structures of the methylzinc methoxide tetramer and of the dimer
(MeZnNPh 2 ) 2 illustrate this well (p. 58). Hydrolysis of an alkyl is probably
generally initiated by adduct formation. The hydrolyses ofMe 3 B and ofMe 3 Al are
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