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

4
Organometallic compounds
of elements of main
groups IV and V
4.1 Introduction
In nearly all their organic derivatives, silicon, germanium, tin and lead are
tetravalent. The tetraalkyls and -aryls are air stable materials which are not
hydrolysed under neutral conditions. Their thermal stability decreases down the
Group in parallel with the trend in metal-carbon bond energies (p. 5 ).
Tetraphenylsilane can be distilled in air at 430°C/760mm. Tetramethylsilane
starts to break up only above 600°C to a complicated mixture of products which
probably arise through radical chain reactions. Liquid Me 4 Pb, however, has been
known to explode even at 90-1 G0°C. Decomposition of tetramethyllead in the
gas phase, begins at only about 2 SO"C giving ethane and lead, together with small
quantities of other hydrocarbons and hydrogen. The existence of free radicals was
first demonstrated by Paneth in 1929 from pyrolysis ofMe 4 Pb in a flow system at
low pressure. The radicals produced removed mirrors of lead, antimony or other
elements forming their methyl derivatives.
It is perhaps in their propensity to oxidation and hydrolysis, however, that the
greatest contrast between Group IV organometallics and those of neighbouring
groups lies. Tetramethylsilane is so inert chemically that it is chosen as an
internat standard for n.m.r. spectroscopy. (It absorbs at higher field than most
organic compounds.) Trimethylaluminium, on the other hand, not only inflames
in air but also reacts explosively with water. Me 3 Al is coordinatively unsaturated;
it is a six electron compound with an empty valence orbital and is hence very
susceptible to nucleophilic attack. Tetramethylsilane behaves as a saturated
compound. Group IV elements show little tendency to increase their coordination
number above four unless they are attached to electronegative atoms or groups.
Although trimethylphosphine also has an octet configuration, it is spontaneously
inflammable. The ease of oxidation of Group V alkyls must be associated
with the presence of a non-bonding pair of electrons on the central atom. The less
basic triaryls are air stable. although controlled oxidation to the oxides Ar 3 MO is
98