Hydrogenation using (Ph3P)3RhCl - Wilkinson's catalyst
(Ph3P)3RhCl, which is generally known as Wilkinson's
catalyst, acts as a homogeneous catalyst for the hydrogenation of alkenes
and alkynes. It does not catalyse the reduction of other organic functional
groups, although it is a reactive compound and does react with
many organic and inorganic reagents. (Ph3P)3RhCl
is know to slightly dissociated (about 5%) into the 14e-electron species
(Ph3P)2RhCl and PPh3 in pure solvents
of low coordinating power (eg: in benzene). (Ph3P)2RhCl
is thought to pick up solvent to generate (Ph3P)2RhCl(S)
which undergoes a series of oxidative addition and elimination reactions
and the equilibrium is displaced.
(Reference 3)
R. Tolman has articulated a rule that seems to govern all transition-metal-mediated catalytic processes:
"Detectable concentrations of diameagnetic organometallic complexes of the transition metals exist only if the central metal atom contains 16 or 18 electrons."
This image describes the Wilkinson's catalytic cycle. Every 15 seconds the next step will be shown.
NB: This is not the only cycle that can be drawn. Some scientists found a five coordinate species [A]: (Ph3P)2RhCl(H2) and proposed an alternative cycle.
Question: Complete the alternative catalytical cycle by yourself, and have a look if you are right.
[A] |
 [A] |
