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

The main Groups IV and V
Table 4.2. Bond energy terms for homonuclear bonds (kJ moi- 1 )
E(C-C)
E(Si-Si)
E(Ge-Ge)
E(Sn-Sn)
358 E(N-N)
220 E(P-P)
189 E(As-As)
151 E(Sb-Sb)
159 E(0-0)
214 E(S-S)
167 E(Se-Se)
142
142
268
159
Values quoted vary from source to source. Those given above
serve only as a guide to trends and should not be accepted as
accurately valid in organometallic compounds.
4.6.2 Organopolysilanes
Many textbooks give the impression that catenation among Group IV elements,
apart from carbon, is rather limited. This is certainly not so as far as
organopolysilanes are concerned. Polymeric compounds Me,Si(SiMe 2 )"SiMe 3
with chain lengths of at least 3000 units have been prepared and cyclic
compounds (Me 2 Si)n which have up to 35 silicon atoms in the ring have been
identified. A wide range of aryl compounds is also known. These materials are not
only thermally stable but in contrast to silicon hydrides and halides, are resistant
to hydrolysis and to oxidation. Polysilanes such as Si 12 Me 26 can be distilled in air
at 300°C. The difference between these organic derivatives and the hydrides is
probably associated with the lower mobility and reactivity of alkyl and aryl
substituents compared with hydrogen.
The most general method of preparation employs the Wurtz reaction, in
which alkyl or arylchlorosilanes react with alkali metals or magnesium, often in
tetrahydrofuran.
2Me,SiCl ~ Me,Si-SiMe, (b.p. 113 °C)
THF
2Me 2 SiCI 2 ~ (Me,Siln + (Me 2 Si) 6 + other rings
THF
polymer
m.p. 2 54 oc
In the early stages of the reaction with Me 2 SiCl 2 , while some chloro compound
still remains, a polymeric species is produced. The Si -Si bonds in this are cleaved
by alkali metals and the resulting metal derivatives react with Me 2 SiCl 2 to
produce cyclic materials. The major product is (Me 2 Si) 6 • Other cyclic compounds
(Me 2 Si)n are also formed, especially under non-equilibrium conditions. Rings from
n = 5 up to n = 24 ha ve been isolated and others up to n = 3 5 identified using
high pressure liquid chromatography.
When neat Me 2 SiCl 2 reacts with sodium metal in an autoclave, polymeric
(Mc 2 Si)n is formed. When heated to 320°C under argon it rearranges to a polymer
which has a backbone of alternating carbon and silicon atoms. This can be drawn
into strands which on heat treatment at 1300oC in vacuo produce fibres of fJ-SiC
of very high tensile strength.
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