chemistry

Wilkinson's catalyst

Wilkinson's catalyst is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wilkinson's catalyst rather than just read about it. In short: Wilkinson's catalyst (chloridotris(triphenylphosphine)rhodium(I)) is a coordination complex of rhodium with the formula [RhCl(PPh3)3], where 'Ph' denotes a phenyl group. It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in tetrahydrofuran or chlorinated solvents such as dichloromethane.

Key takeaways

Wilkinson's catalyst belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Wilkinson's catalyst to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wilkinson's catalyst from memory before moving on to harder problems.

Reference excerpt

Wilkinson's catalyst (chloridotris(triphenylphosphine)rhodium(I)) is a coordination complex of rhodium with the formula [RhCl(PPh3)3], where 'Ph' denotes a phenyl group. It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in tetrahydrofuran or chlorinated solvents such as dichloromethane. The compound is widely used as a catalyst for hydrogenation of alkenes. It is named after chemist and Nobel laureate Sir Geoffrey Wilkinson, who first popularized its use. Historically, Wilkinson's catalyst has been a paradigm in catalytic studies leading to several advances in the field such as the implementation of some of the first heteronuclear magnetic resonance studies for its structural elucidation in solution (31P), parahydrogen-induced polarization spectroscopy to determine the nature of transient reactive species, or one of the first detailed kinetic investigation by Halpern to elucidate the mechanism. Furthermore, the catalytic and organometallic studies on Wilkinson's catalyst also played a significant role on the subsequent development of cationic Rh- and Ru-based asymmetric hydrogenation transfer catalysts which set the foundations for modern asymmetric catalysis.

Structure and basic properties According to single crystal X-ray diffraction the compound adopts a slightly distorted square planar structure. In analyzing the bonding, it is a complex of Rh(I), a 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). In contrast, IrCl(PPh3)3 undergoes cyclometallation to give HIrCl(PPh3)2(PPh2C6H4), a coordinatively saturated Ir(III) complex that is not catalytically active.

Synthesis Wilkinson's catalyst is usually obtained by treating rhodium(III) chloride hydrate with an excess of triphenylphosphine in refluxing ethanol. Triphenylphosphine serves as both a ligand and 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

Catalytic applications Wilkinson's catalyst is best known for catalyzing the hydrogenation of alkenes with molecular hydrogen. The mechanism of this reaction involves the initial dissociation of one or two triphenylphosphine 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, e.g.:

In terms of their rates of hydrogenation, the degree of substitution on the alkene substrate is the key factor, since the rate-limiting step in the mechanism is the insertion into the olefin which is limited by the severe steric hindrance around the metal center. In practice, terminal and disubstituted alkenes are good substrates, but more hindered alkenes are slower to hydrogenate. The hydrogenation of alkynes is troublesome to control since alkynes tend to be reduced to alkanes, via intermediacy of the cis-alkene. Ethylene reacts with Wilkinson's catalyst to give RhCl(C2H4)(PPh3)2, but it is not a substrate for hydrogenation.

Related catalytic processes Wilkinson's catalyst also catalyzes many other hydrofunctionalization reactions including hydroacylation, hydroboration, and hydrosilylation of alkenes. Hydroborations have been studied with catecholborane and pinacolborane. It is also active for the hydrosilylation of alkenes. In the presence of strong base and hydrogen, Wilkinson's catalyst forms reactive Rh(I) species with superior catalytic activities on the hydrogenation of internal alkynes and functionalized tri-substituted alkenes.

Reactions RhCl(PPh3)3 reacts with carbon monoxide to give bis(triphenylphosphine)rhodium carbonyl chloride, trans-RhCl(CO)(PPh3)2. The same complex arises from the decarbonylation of aldehydes:

RhCl(PPh3)3 + RCHO → RhCl(CO)(PPh3)2 + RH + PPh3 Upon stirring in benzene solution, RhCl(PPh3)3 converts to the poorly soluble red-colored dimer [RhCl(PPh3)2]2. This conversion further demonstrates the lability of the triphenylphosphine ligands. In the presence of base, H2, and additional triphenylphosphine, Wilkinson's complex converts to hydridotetrakis(triphenylphosphine)rhodium(I), HRh(PPh3)4. This 18e complex is also an active hydrogenation catalyst.

See also Rhodium-catalyzed hydrogenation

References

Illustrations

Worked examples

Example 1 — a first encounter with Wilkinson's catalyst

Start with the simplest possible case. Write down what Wilkinson's catalyst claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wilkinson's catalyst before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wilkinson's catalyst ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wilkinson's catalyst

In research: Wilkinson's catalyst appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wilkinson's catalyst in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wilkinson's catalyst is common in secondary-school and first-year university syllabi. It links to neighbouring topics Catalysts, Coordination complexes, Homogeneous catalysis, so understanding it makes those chapters shorter.
In everyday life: Look for Wilkinson's catalyst outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Wilkinson's catalyst in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Wilkinson's catalyst means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Wilkinson's catalyst out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wilkinson's catalyst in simple terms?

Why does Wilkinson's catalyst matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wilkinson's catalyst?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wilkinson's catalyst.