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

3
Organometallic compounds
of elements of the first
three periodic groups
3.1 Introduction
In this chapter the organic compounds of the first three Periodic Groups are
discussed. The alkali metals, Groups IIA and IIB, boron, aluminium and Group
IIIB are included. Scandium, yttrium and lanthanum are best considered with the
lanthanides and are therefore covered in Chapter 13. Although copper, silver and
gold are strictly transition elements, some oftheir organic chemistry in oxidation
state + l relates quite well to that of the main groups. It is therefore mentioned
briefly here.
The heavy alkali metals K, Rb and Cs form essentially ionic derivatives. These
are extremely reactive; caesium alkyls, for instance, are such strong bases that
they can even deprotonate alkanes. Consequently their organic chemistry is less
developed than that of the lighter elements, lithium and sodium. These small
electropositive atoms enter into bridge bonding with organic groups; this
property also extends in particular to beryllium and magnesium in Group II and
to aluminium in Group III. Some copper and silver alkyls and aryls also possess
this structural feature.
Organomagnesium compounds, especially the Grignard reagents, RMgHal,
have played a key role in organic and organometallic synthesis for nearly 90
years. Of particular value is their ability to add to polar groups such as :::::. C=O
and -C-N. Organoberyllium compounds have not yet found any significant
synthetic applications, largely because beryllium oxide, formed by their combustion,
is toxic by inhalation as dust. They are generally less accessible, less stable
and less reactive than the derivatives of magnesium and lithium. Their behaviour
is less complicated by side-reactions than that of Grignard reagents. Even less is
known of the organic chemistry of calcium, strontium and barium. Organozinc,
cadmium and mercury compounds, however, have been known for over a
century and are stiU quite widely used in synthesis.
The general reactivity of the organic compounds of Group Il lies in the
sequence Ba > Sr > Ca > Mg > Be ~ Zn > Cd > Hg. This order follows bond
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