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

Complexes of alkenes
also forms analogous anionic complexes, but the ethene is oxidized in aqueous
media to ethanol, forming the hasis of the Wacker process (p. 380). Under other
conditions halogen bridged dimers or 17 3 -allyl complexes (p. 2 61) are formed.
heat in
p d C L 2 --::ccPh--:-C::-:-N -.
The use of the benzonitrile complex illustrates an approach often adopted in the
preparation of labile derivatives. This is to take as starting material a complex
from which the ligands can be even more readily displaced. An excess of the
incoming ligand can be used, particularly if it is volatile and can be pumped away
at the end of the reaction.
Use of reducing agent: The starting materials for the synthesis of organometallic
compounds which are readily available are halides, acetylacetonates etc. In many
cases it is necessary to include a reducing agent to remove the halide and to
provide electrons to the metal centre (p. 166 ). For rhodium and iridium, the
reducing medium is provided by an alcohol.
MCL 3 .3H 20 + Z-cyclooctene
aq.
The dimeric products are useful starting materials for the preparation of other
complexes, as the alkenes are readily displaced. The Z-cyclooctene compounds
can be stored indefinitely at ooc under nitrogen.
~~
~oleum
o•c
CL
1
~))
1,5-cyclooctadiene
[(1,5-COD)RhCL h
Stronger reducing agents such as metals, aluminium alkyls or Grignard reagents
are often required, especially for elements of the First Transition Series.
Mn or
NiCL2+1,5-cyclooctadiene ----.
Et2AlOEt
Tetrahedral Ni,
18e
239