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

Transition metal chemistry
Pd(PPhBu~ ) 2 which shows little tendency to add further ligands or undergo
addition reactions.
The complexes ML 4 add molecules X-Y in which two ligands are lost and a
square planar product results:
ML 4 ~ ML, ~ML 2 + XY--> MX(Y)L 2
Acids HX react with Pt(PPh,), to give salts [PtH(PPh,),]+ x- if x- is weakly
coordinating, but otherwise Ph,P is displaced, yielding neutra! trans derivatives:
HX -PPh 3
Pt(PPh,),~ [PtH(PPh,),]+x- ~ trans FtHX(PPh,) 2 X=CN, CI
KOH +PPh 3
Some reactions of Pd(PPh,) 4 are given in Fig. 5.18. The platinum derivative
behaves similarly. Sulphur dioxide acts as a Lewis acid. The reactions with alkyl
halides usually follow the SN2 mechanism, although free radical pathways can
also participate. The reactions of aryl halides, which are normally rather
unreactive towards nucleophiles are of interest. The palladium complexes
catalyse the arylation of alkenes (the Heck reaction) by the following cycle:
~
-HX
~-ArCH-CH2
(c) WILKINSON'S COMPLEX- CATALYTIC HYDROGENATION OF ALKENES ANO
ALKYNES. Wilkinson's compound, RhCl(PPh,),, is readily prepared by heating
RhCl3'3Hp under reflux in ethanol with an excess of triphenylphosphine. It is
usually obtained as red-violet crystals, but there is also a metastable orange
modification. The molecule is distorted from ideal square planar geometry,
presumably on account of steric interactions. Many of its reactions involve
addition to the coordinatively unsaturated rhodium centre. The products are
sometimes 18-electron species, but often loss of one phosphine ligand occurs, so
that a 16-electron derivative results (as with CO or C 2 H 4 ) (Fig. 5.19).
Dilute solutions ("' 10-' moi dm- 3 ) of Wilkinson's complex in organic
solvents such as ethanol/benzene actively catalyse the hydrogenation of alkynes
and alkenes at ambient temperature and pressure (1 atm, H 2 ). Traces of oxygen
must be excluded. Hydrogenation rates are dependent on steric effects. The
following general order has been established: 1-alkenes > cis-2-alkenes > trans-
2-alkenes > trans-3-alkenes. An exception is ethene itself, which forms a rather
stable complex (Fig. 5.19) so that higher temperatures are required. Arenes,
esters, ketones, carboxylic acids, amides and nitro compounds are unaffected, but
aldehydes are slowly decarbonylated.
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