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

General reactions of transition metal complexes
H
L.,,, 1 _..H
'Rn
Cl~I'L
L
co
-L
RCHO
(-RH)
Fig. 5.19 Some reactions of Wilkinson's compound, L,RhCI. (L = PPh, ).
Tolman proposed two general rules relating to organometallic reactions and
homogeneous catalysis.
Rule 1: The only metal-containing species which can exist in significant
concentration possess either 16 or 18-electron configurations.
Rule 2: Organometallic reactions, including catalytic o nes, proceed by elementary
steps which involve 16 or 18-electron intermediates.
Some exceptions have already been noted; with Pd or Pt. for example, the 16-
electron square planar complexes behave to some extent as if they are 'saturated'
and 14-electron species are sometimes observed and are likely reaction intermediates.
Moreover odd-electron free radical intermediates also sometimes intervene.
Nevertheless the rules provide a very useful starting point for formulating possible
mechanisms for catalytic cycles.
Hydrogenation using Wilkinson's catalyst has been studied in great detail and
some features are stil! not understood. A simplified scheme, based on Tolman's
rules is given in Fig. 5.20.
The dihydride L 3 RhH 2 Cl arises from cis addition of H 2 to L 3 RhCl and has the
structure shown in Fig. 5.19. The catalytically active species, however, has only
two phosphine ligands and may be five-coordinate. Although isomer A is oflower
energy than B, the latter is thought to be involved in the actual hydrogenation.
Addition of alkene is followed by hydrogen transfer, the slowest stop of the whole
cycle, to give a rhodium alkyl hydride (C). Elimination of alkane from C is very
fast. A problem with the scheme appears at this point. Does C give a 14-electron
species L 2 RhCl (in contravention of the rules) or does a ligand enter? It has been
suggested that the L 2 RhCl is stabilized by coordination of a solvent molecule; this
is supported by the observation that hydrogenation is about twice as fast in
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