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

Tetra-alkyls and -aryls of Group IV
stil! readily achieved chemically. Bonds between carbon and Group V elements
are normally stable to hydrolysis under neutra! conditions. Cleavage of M -C
bonds by acids, which involves electrophilic attack at carbon. becomes increasingly
facile down both Group IV and Group V. This is influenced by the polarity as
well as by the strength of the bond.
In addition to tetraorgano- derivatives R 4 M, the Group IV elements form a
large number of compounds R 11 MX 4 _ 11 , where X is an electronegative substituent.
The halides are very useful starting materials for the preparation of other species.
The tin and lead halides, especially the fluorides, tend to associate through
halogen bridges in the solid state. The metal atom can also increase its
coordination number to five or six by complex formation with halide ions or with
other Lewis bases.
Organic derivatives of Group IV tind applications in diverse areas. Especially
important are the silicone polymers, organotin compounds as stabilizers for
plastics and as biocides and organolead antiknock agents for petrol.
4.2 Tetra-alkyls and -aryls of the elements of Group IV
4.2 .1 Preparation
The first organosilicon compound to be prepared was tetraethylsilane. It was
obtained by Friedel and Crafts in 1863 by reacting diethylzinc with silicon
tetrachloride at 150°C. Nowadays in the laboratory. however. tetraorgano
derivatives of Si, Ge and Sn are conveniently prepared by reacting the
tetrachloride with excess Grignard or organolithium reagent. This is straightforward
for silicon and germanium. Tin(IV) chloride forms a sparingly soluble
adduct with diethyl ether, the usual solvent for Grignard reactions, much heat
being evolved. It is advisable to add the SnCl 4 to excess Grignard solution to
minimize the formation ofpartially substituted products R 1 SnCI 1 and R,SnCl. To
aid the addition and to reduce the vigour of the reaction it is therefore expedient
to dissolve the tin halide in benzene or toluene.
Organoaluminium compounds can also be used to transfer the organic groups.
With tin halogen bridged complexes such as R 1 SnCl 1 .AICI, R,SnCl.AICI, are
formed which hinder further alkylation. This can be prevented by addition of
donors such as NaCl or an amine which complex with the aluminium halide as it
is formed.
4R ,Al+ 4NaCI -t 3SnCI. -----. 3R 4 Sn + 4NaAICI,
By careful addition of a stoichiometric quantity of Grignard reagent to the
halide, or better the alkoxide, stepwise attachment of organic groups can often be
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