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

The main Groups IV and V
4.8 pTC-JJTC Bonding in compounds of Groups IV and V
Multiple bond formation between the elements ofthe First Short Period C, N and O
is a common phenomenon. Relative to single bonds, multiple bonds between
these elements are much stronger than those which involve o ne or more elements
from !ater Periods. While (2p-2p)n overlap is rather strong, similar overlap
involving p orbitals of higher principal quantum number is less effective. Thus
C0 1 and Me 1 CO monomeric, whereas Si0 1 and (Me 1 Si0)" polymeric (p.112). It is
instructive to note that 2E(C-C)= 712kJmol- 1 , whereas E(C=C) is only
602 kJ moi- 1 • Ethene is thermodynamically unstable with respect to polyethene.
While ethene does not polymerize under normal conditions, it does so very rapidly
when catalysed (p. 371 ). Ethenes RCH =CH 2 with electron withdrawing substituents
(R = CN, C0 1 Et or Ph) are especially susceptible to polymerization initiated
by free radicals or by traces of acids or bases. Can species such as R 1 Si=SiR 1 or
R 2 Si=CR; be obtained if pathways to polymerization can be blocked? If 2E(Si­
Si) = 440 kJ moi- 1 and E(Si =Si) = 314 kJ moi- 1 , the prospect from thermodynamics
does not look very hopeful. Nevertheless many attempts ha ve been made
to prepare such compounds and recently these have met with some success.
Pyrolysis of dimethylsilacyclobutane at 600°C gives a mixture of ethene and
dimethylsilaethene. The latter species has been detected in the gas phase by mass
spectrometry. It has also been trapped in a solid argon matrix at 10 K and its
infrared spectrum measured. It is very reactive, dimerizing readily and adding to
dienes in a Diels-Alder reaction:
CH2
Il +
CH2
The introduction of very bulky substituents hinders dimerization and in the
following example also prevents intermolecuiar rearrangement of small Si 3 rings
to larger oligomers (Ar= 2. 4, 6-trimethyl-phenyl).
hll
Ar2Si-SiAr2 ----+ Ar2Si=SiAr2
\
/ Cyclohexane
Si
Ar 2
m.p. 272°C
Yellow crystals
m.p. 2T5°C
The bulkier the substituents the more sta bie the silene becomes. While Me 2 Si=
SiMe 2 has been observed only in a frozen matrix at low temperatures, Bu~Si=
SiBu~ persists for a few hours under ambient conditions. Ar 2 Si=SiAr 1 and
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