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

Diene complexes
with retention. (In fact there are two inversion steps, o ne in the formation of the
allylpalladium intermediate and another in its displacement.) Good control ofthe
stereochemical course of these substitutions is therefore possible.
8.2 Diene complexes
The conjugated diolefin l, 3-butadiene exists almost entirely in the planar s-trans
conformation at 298 K.
LlG,j:/6 kJ mol-1
LlG~2~mol- 1
s-trans
s-cis
The s-trans conformer is about 12 kJ moi- 1 more stable than the s-cis. Nevertheless
it is the cis structure which 1, 3-diene ligands normally adopt when they are
174-bonded to transition elements. In this way better overlap between C(2p)n
orbitals of the ligand and the metal orbitals is achieved.
The largest group of diene complexes which have been investigated are iron
carbonyl compounds. When l. 3-butadiene reacts with Fe 2 (C0) 9 , the initial
product is a labile 17 2 -complex, which readily loses carbon monoxide to yield (174-
C4H6)Fe(C0)3. Butadiene(tricarbonyl)iron is a yellow, essentially air stable
complex, m.p. l9°C. It was first prepared by Reihlen in 19 30 by heating Fe(C0) 5
with butadiene in a tube under pressure, but this was an isolated discovery which
did not arouse much interest at the time. On account of the ease of preparation
and handling of these diene complexes, however, an enormous amount of work
has been do ne in this are a since the late 1 9 SOs. Only modest precautions are
required to protect solutions from oxidation and some operations can be carried
out even in air.
While l, 3-butadiene itself is a gas at room temperature, substituted dienes are
liquids or solids. In general, therefore, pressure vessels are not required in the
preparation of the 17 4 -diene complexes. Starting materials include Fe(C0) 5 ,
Fe 2 (C0) 9 or Fe 3 (C0) 12 • The crucial initial step in each case is the formation ofthe
coordinatively unsaturated 16-electron intermediate Fe(C0) 4 • This can be
induced thermally or, in the case of Fe(CO),. by irradiation.
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