Wilkinson's catalyst, chloridotris(triphenylphosphine)rhodium(i), is a rhodium compound used primarily for the hydrogenation of alkenes, synthesized by reacting rhodium(iii) chloride with triphenylphosphine. The catalyst has a square planar structure and operates through a mechanism involving the dissociation of ligands and subsequent hydrogen activation, facilitating selective hydrogenation of less substituted or less hindered double bonds. Notably, it exhibits selectivity influenced by sterics and functionality, providing unique outcomes in hydrogenation reactions.

1. 4. Wilkinson’s catalyst
Dr. Shivendra Singh
UGC-NET-JRF, PhD-IIT Indore

2. …Wilkinson’s catalyst
 Wilkinson's catalyst is the common name for chloridotris(triphenyl-
phosphine)rhodium(I), with the formula [RhCl(PPh3)3].
 It is a red-brown colored solid that is soluble in hydrocarbon solvents such
as benzene, and more so in THF or chlorinated solvents.
 The compound is widely used as a catalyst for hydrogenation of alkenes.
 Named after chemist and Nobel laureate Sir Geoffrey Wilkinson, who
first popularized its use.
[RhCl(PPh3)3]

3. …Wilkinson’s catalyst: Synthesis
 Wilkinson's catalyst is usually obtained by treating rhodium(III) chloride
hydrate with an excess of triphenylphosphine in refluxing ethanol.
 Triphenylphosphine serves as a two-electron reducing agent that
oxidizes itself from oxidation state (III) to (V). In the synthesis, three
equivalents of triphenylphosphine become ligands in the product, while
the fourth reduces rhodium(III) to rhodium(I).
RhCl3(H2O)3 + 4 PPh3 → RhCl(PPh3)3 + OPPh3 + 2 HCl + 2 H2O
[RhCl(PPh3)3]

4. …Wilkinson’s catalyst: Structure
 According to single crystal X-ray diffraction, the compound adopts a
slightly distorted square planar structure.
 It is a complex of Rh(I), d8 transition metal ion.
 From the perspective of the 18-electron rule, the four ligands each provides
two electrons, for a total of 16-electrons. As such the compound is
coordinatively unsaturated, i.e. susceptible to binding substrates (alkenes
and H2).
[RhCl(PPh3)3]
45Rh: [Kr] 4d8 5s1

5. …Wilkinson’s catalyst: Application
 The mechanism involves the initial
dissociation of one or two PPh3
ligands to give 14- or 12-electron
complexes, respectively, followed by
oxidative addition of H2 to the metal.
 Subsequent π-complexation of
alkene, migratory insertion
(intramolecular hydride transfer or
olefin insertion), and reductive
elimination complete the formation
of the alkane product.
[RhCl(PPh3)3]
π-complexation
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6. …Wilkinson’s catalyst
Above mechanism is supported by following observations;
1.The rate of reaction decreases when excess of PPh3 is added; indicating the initial
dissociation of one of the PPh3 ligand before dihydrogen activation.
3. Though ethylene cannot be hydrogenated in presence of Wilkinson's catalyst under
normal conditions, hydrogen transfer can be achieved with preformed dihydrido
complex.
[RhCl(PPh3)3]
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2. It is observed that strong π-acids like ethylene act as poisons by binding strongly
with the electron rich Rh metal center and inhibit hydrogenation.

7. …Wilkinson’s catalyst
4. The rates of hydrogenations decrease with increase in the alkyl group substitution
on double bond mirroring their relative binding affinities to the metal center. It is
also partly due to steric factors.
5. There is minimal scrambling of H/D in the product, when an equimolar mixture
of H2 and D2 are used.
[RhCl(PPh3)3]
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8. …Wilkinson’s catalyst: Examples
Less substituted and sterically less hindered
double bonds are selectively hydrogenated.
Exocyclic double bonds are selectively
hydrogenated over endocyclic.
Cis alkenes are reduced rapidly than
trans alkenes.
Isolated double bonds are rapidly
hydrogenated over conjugated dienes.
[RhCl(PPh3)3]
1.
2.
3.
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9. Terminal alkynes are hydrogenated more rapidly
than terminal alkenes. The selectivity can be
enhanced by using acidic alcoholic co-solvents.
Functional groups like C=O, C=N, NO2, Aryl,
CO2R etc., are unaffected. The compatibility
of WC with polar multiple bonds shows the
metal hydride bond is primarily covalent in
character.
Unsaturated substrates containing polar
functionality are hydrogenated more rapidly. It
may be due to easy coordination of olefin to the
catalyst that is assisted by polar functional
group.
…Wilkinson’s catalyst: Examples
[RhCl(PPh3)3]
5.
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10. …Wilkinson’s catalyst: Examples
 Hydrogenation of Maleic acid with D2 give meso compound exclusively.
 Whereas, with fumaric acid, a racemic mixture is formed.
[RhCl(PPh3)3]
Malic acid
Hydrogenation of Maleic acid or Fumaric acid with D2 in presence of WC is
diastereoselective.
8.
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11. It is observed that hydrogenation of alkynes
give cis alkenes as major products.
The diastereoselectivity of Wilkinson's
hydrogenation is evident in the
formation of endo product as major
one with the following bicyclic
system. The catalyst binds to the
double bond from the least hindered
exo face of the bicyclic system that is
followed by syn addition of two
hydrogen atoms.
…Wilkinson’s catalyst: Examples
[RhCl(PPh3)3]
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12. Since Rh(I) is a low valent and coordinately unsaturated metal and can bind to CO
strongly, the Wilkinson's catalyst can be used to bring about decarbonylation of
aldehydes through oxidative addition route.
…Wilkinson’s catalyst: Examples
The process is non-catalytic, since the catalyst cannot be regenerated.
[RhCl(PPh3)3]
11.
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13. Example
12. Hydrogenolysis of O-benzyl is not observed with WC. Other hydrogenation cleaves
the O-benzyl bond.

14. …Wilkinson’s catalyst: Summary
[RhCl(PPh3)3]
1. Synthesis
2. Mechanism
 Homogeneous catalytic hydrogenation
 Evidence supporting mechanism: 5
3. Applications: 12

15. Next: Lindlar’s & Adam’s catalyst
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