Wilkinson catalyst: Structure, CAS Number, Properties and Uses

Wilkinson's catalyst, RhCl(PPh3)3 (CAS 14694-95-2, PubChem CID 84599), is a rhodium(I) hydrogenation catalyst that is 11.1% rhodium by weight. It hydrogenates simple and disubstituted alkenes while fully dissolved in solution, at 25°C and 1 atmosphere of hydrogen gas, a homogeneous alternative to solid-surface catalysts. Its square-planar structure, five-step catalytic cycle, preparation, commercial form and hazard classification follow below, section by section.

What Is Wilkinson's Catalyst?#

Wilkinson's catalyst is chlorotris(triphenylphosphine)rhodium(I), a square-planar rhodium complex with the formula RhCl(PPh3)3 and CAS number 14694-95-2. PubChem catalogs the compound under CID 84599 and gives its systematic name as "rhodium;tris(triphenylphosphane);chloride," a depiction that carries a spurious -1 charge from PubChem's internal ionic-fragment convention; chemists write the neutral molecule as RhCl(PPh3)3, the conventional formula used throughout this page. Wilkinson's catalyst is one of the platinum group metal compounds and catalysts cataloged with a dedicated CAS registry entry on this site.

PubChem's synonym list for CID 84599 also carries a second CAS candidate, 16592-65-7. No manufacturer or reference page corroborates that number: Wikipedia, the American Chemical Society's Molecule of the Week and Sigma-Aldrich all cite 14694-95-2 only. Treat 14694-95-2 as the sole house CAS for Wilkinson's catalyst and 16592-65-7 as an unconfirmed synonym-list artifact.

Wilkinson's Catalyst Structure and Bonding#

Wilkinson's catalyst adopts a slightly distorted square-planar geometry around a d8, 16-electron rhodium(I) center, per Wikipedia's Wilkinson's catalyst entry and the American Chemical Society's Molecule of the Week profile. One chloride ion and three triphenylphosphine ligands fill the four coordination positions, and the resulting complex is coordinatively unsaturated: with only 16 valence electrons at rhodium instead of the 18 that would fill every orbital, the metal keeps one site open to bind hydrogen and alkene substrates in turn. That open site is what makes the complex catalytically active rather than a stable, unreactive coordination compound.

X-ray crystallography shows that the complex actually crystallizes as two distinct allotropes, an orange form and a red form, each independently structure-determined as a slightly distorted square-planar Rh(I) complex in the same study, according to Bennett and Donaldson's 1977 crystal-structure paper in Inorganic Chemistry (volume 16, pages 655 to 660). Both polymorphs share the same coordination geometry described above; the color difference reflects packing and minor geometric distortion in the solid state rather than a different oxidation state or ligand set.

One of the three triphenylphosphine ligands dissociates reversibly in solution, opening the coordination site that starts the hydrogenation cycle described under Uses, below.

Rhodium Content of Wilkinson's Catalyst#

Rhodium makes up 11.1% of Wilkinson's catalyst by mass (102.90549 g/mol of rhodium in a 925.2 g/mol complex). Rhodium's standard atomic weight, 102.90549 g/mol, known to eight significant figures, divided by the compound's 925.2 g/mol molecular weight (PubChem CID 84599) gives 102.90549 / 925.2 = 0.1112, or 11.1 wt% Rh. That fraction is the basis for pricing a batch of catalyst against its contained rhodium value rather than against a flat per-gram catalog number, since rhodium is the metal whose market the compound's cost tracks. Current pricing for the metal itself is on DailyPlatinum's rhodium price today page.

Physical and Chemical Properties#

Wilkinson's catalyst is a red-brown (burgundy) solid that is insoluble in water and soluble in dichloromethane, chloroform and benzene. According to ChemicalBook's aggregated technical data sheet, the solid decomposes at 245 to 250°C rather than melting sharply, and carries a density of about 1.36 to 1.38 g/cm3, figures cross-checked against this site's own chemistry registry but sourced single-handedly to that aggregator. The same data sheet reports that the complex is air-sensitive in solution, oxidizing to an inactive rhodium(III) oxide species on prolonged air exposure, while the solid form is bench-stable; suppliers recommend storing it in the dark under an inert atmosphere at room temperature.

How Is Wilkinson's Catalyst Made?#

Wilkinson's catalyst is made by refluxing rhodium(III) chloride trihydrate with excess triphenylphosphine in ethanol. The precursor matters: rhodium(III) chloride trihydrate (RhCl3·3H2O, CAS 13569-65-8, per Wikipedia's Rhodium(III) chloride entry) is water- and alcohol-soluble, while anhydrous rhodium(III) chloride (RhCl3, CAS 10049-07-7) is insoluble in water and acids and essentially inert, useless as a starting point for this synthesis. Only the hydrate dissolves readily enough to react in solution.

The preparation follows four steps:

1. Dissolve rhodium(III) chloride trihydrate in hot ethanol.
2. Add a large excess of triphenylphosphine (PPh3); the excess is stoichiometrically necessary, not merely a kinetic convenience, because PPh3 serves two roles at once, the ligand that builds the final complex and the reducing agent that converts rhodium from the +3 to the +1 oxidation state.
3. Reflux the mixture, during which rhodium(I) coordinates three PPh3 ligands and one chloride ion.
4. Isolate the product as a red-brown solid on cooling.

This route, refluxing rhodium(III) chloride hydrate with excess triphenylphosphine in ethanol, is the standard preparation documented for the compound, and it is also the procedure published as a named, citable method rather than only a discovery-paper description: Osborn, Wilkinson and Mrowca, "Tris(triphenylphosphine)halorhodium(I)," Inorganic Syntheses, volume 10 (1967), pages 67 to 71.

What Is Wilkinson's Catalyst Used For?#

Wilkinson's catalyst is used for homogeneous alkene hydrogenation, and also for hydroboration, hydroacylation and hydrosilylation. It was the first homogeneous catalyst to hydrogenate alkenes at rates comparable to the best heterogeneous catalysts of its era, working rapidly at 25°C and 1 atmosphere of hydrogen gas, according to the University of Bristol's Molecule of the Month (July 2013) and the American Chemical Society's Molecule of the Week. Laboratory procedures commonly run the catalyst at 1 to 5 mol% rhodium loading for alkene hydrogenation, selective for terminal and cis-disubstituted alkenes and much slower on hindered or tetrasubstituted substrates, per Wikipedia and the ACS Molecule of the Week profile; the catalyst does not hydrogenate ethylene productively. As a dissolved, homogeneous catalyst it belongs to a different family from the fixed-bed PGM catalysts in chemistry used in industrial reactors and vehicle exhaust systems.

The rhodium(I) phosphine framework covers four related reactions, alongside its primary hydrogenation role:

• Alkene hydrogenation: reduces simple, unhindered alkenes at 25°C and 1 atm H2, mild enough to leave many other functional groups untouched.
• Hydrosilylation: adds Si-H bonds across alkenes, used where platinum catalysts such as the Speier catalyst or the Karstedt catalyst give inadequate selectivity.
• Hydroboration: adds B-H bonds across alkenes through the same open-site mechanism.
• Hydroacylation: forms a new carbon-carbon bond from an aldehyde C-H bond and an alkene.

Beyond hydrogenation, ChemicalBook's aggregated reaction data lists the aldehyde decarbonylation named after Tsuji and Wilkinson, regio- and stereoselective allylic substitution, alkyne hydrophosphorylation, Heck-type coupling with alpha,beta-unsaturated esters, alkyne arylation and reductive deprotection of silyl groups as further reactions run on the same rhodium(I) center. Substrate scope has a documented limit: hydrogenation of alkynes is poorly controlled, over-reducing to alkanes through a cis-alkene intermediate, and more hindered or tetrasubstituted alkenes react far more slowly than terminal and disubstituted ones, limited by a migratory-insertion step that slows as steric crowding around rhodium increases, per Wikipedia's Wilkinson's catalyst entry. That exact gap in substrate scope is what Crabtree's catalyst was later developed to fill, covered in the comparison below.

Catalytic Cycle#

The Wilkinson's catalyst hydrogenation cycle runs through five steps: phosphine dissociation, oxidative addition of hydrogen, alkene coordination, migratory insertion and reductive elimination.

1. Dissociate one triphenylphosphine ligand from the 16-electron, four-coordinate complex, opening a coordination site at rhodium.
2. Add hydrogen gas oxidatively across that open site, oxidizing rhodium from Rh(I) to Rh(III) and forming a six-coordinate dihydride.
3. Coordinate the alkene substrate at the site vacated by the dissociated phosphine.
4. Migrate one rhodium-bound hydride onto the coordinated alkene, forming a rhodium-alkyl bond; this migratory-insertion step is rate-limiting, slowed by steric crowding around rhodium.
5. Eliminate reductively: the alkyl group combines with the remaining hydride to release the saturated hydrocarbon product and regenerate the original 16-electron Rh(I) complex, ready to re-enter the cycle.

Jack Halpern's kinetic studies, as summarized in secondary literature on the mechanism, established that the cycle proceeds through an initial 14-electron species formed by phosphine dissociation before hydrogen adds oxidatively, and that adding excess free triphenylphosphine to the reaction measurably slows the observed hydrogenation rate, consistent with that dissociation step and with migratory insertion, not phosphine loss, as the turnover-limiting step.

Wilkinson's Catalyst vs Crabtree's Catalyst#

Wilkinson's catalyst hydrogenates simple and disubstituted alkenes rapidly at 25°C and 1 atm H2, but cannot touch the tetrasubstituted alkenes that Crabtree's catalyst hydrogenates. The Crabtree catalyst, an iridium(I) cationic complex discovered by Robert Crabtree and graduate student George Morris in the 1970s, hydrogenates tetrasubstituted and other hindered olefins at high turnover frequency through directed hydrogenation, a substrate class neither Wilkinson's catalyst nor the related Schrock-Osborn catalysts can reach.

Both catalysts are homogeneous, phosphine-containing hydrogenation catalysts developed within about a decade of each other. Chemists choose between them by substrate: Wilkinson's catalyst for simple alkenes, Crabtree's catalyst for the hindered substrates Wilkinson's catalyst leaves untouched.

Commercial Forms and Price#

Suppliers sell Wilkinson's catalyst as a solid reagent in gram to multi-gram laboratory quantities, priced per gram rather than by the troy ounce used for bullion PGMs. Because the complex is 11.1% rhodium by weight, its price tracks the rhodium market more closely than a flat catalog number, and volume buyers typically price a batch against contained-rhodium value rather than a fixed per-gram rate. Johnson Matthey lists the compound as a named catalog product, Rh-100, and Heraeus Precious Metals carries it alongside Sigma-Aldrich, Strem and American Elements; commercial material is sold as a free-flowing solid at 97 to 99.9% purity, with trace-metals-basis grades up to 99.98%, and refiners also produce it in bulk for catalyst recycling and reclaim, not only for lab-scale research supply.

Pack sizes and list prices vary by supplier. ChemicalBook's aggregated supplier-price listing, dated 30 April 2026, recorded Sigma-Aldrich catalog number 199982 at $285 for 1 gram and $1,320 for 5 grams; TCI Chemical catalog number T0931 (greater than 98.0% purity) at $159 for 1 gram and $363 for 5 grams; and Strem Chemicals catalog number 45-0650 (99% purity, marketed explicitly as "Wilkinson's catalyst") at $85 for 250 milligrams. These are historical list prices at the stated date, not a live quote.

Safety and Hazard Classification#

PubChem records no GHS signal word and no hazard codes on file for Wilkinson's catalyst; consult the supplier safety data sheet before handling. GHS classification reported to PubChem: no signal word, no hazard statement codes, zero notifications on file for CID 84599. That absence is not a certification of safety; it means no classification has been registered for this specific compound, and laboratories still handle the solid under standard precautions, gloves and fume-hood practice, with its dust not inhaled.

Beyond PubChem's own record, ChemicalBook's aggregated regulatory data lists the compound as active on the US TSCA inventory and records its EU REACH registration status as active, though the tonnage band behind that registration is not established in the cited source. The same aggregator found no confirmed UN number or DOT hazard class specific to the neat solid, consistent with material that is not classified as a dangerous good in the small laboratory quantities typically shipped; treat this as ChemicalBook's reported finding rather than an independently verified transport classification. No compound-specific occupational exposure limit exists for Wilkinson's catalyst itself. As background regulatory context only, the US CDC/NIOSH 1988 OSHA PEL Project record sets a permissible exposure limit of 0.001 mg/m3 (as Rh) and an ACGIH threshold limit value of 0.01 mg/m3 for soluble rhodium compounds generally, a category that includes the soluble rhodium(I) center in this complex even though no SDS ties that figure to this specific catalyst.

Where This Page Stops: Rhodium Jewelry Plating and Platinum Drugs#

This page covers Wilkinson's catalyst as a laboratory and industrial reagent only, not as rhodium jewelry plating or as a platinum-based drug. Rhodium plating uses a different rhodium salt bath, not this complex, to give white gold and sterling silver jewelry a bright, tarnish-resistant finish, a decorative use with no catalytic function. Platinum-based pharmaceuticals such as cisplatin are a distinct pharmacological subject, covered elsewhere on this site without dosing or clinical content. Wilkinson's catalyst is also unrelated to the fixed-bed three-way catalyst used in vehicle exhaust systems, which runs heterogeneous, not homogeneous, chemistry.

Related PGM Compounds#

Wilkinson's catalyst shares its rhodium(I) chemistry with rhodium(III) chloride, its own precursor, and with the site's other named hydrogenation and cross-coupling catalysts.

History#

Wilkinson's catalyst was popularized in the 1960s by Sir Geoffrey Wilkinson. The compound's discovery is documented in a named, citable primary paper: Osborn, Jardine, Young and Wilkinson, "The preparation and properties of tris(triphenylphosphine)halogenorhodium(I) and some reactions thereof including catalytic homogeneous hydrogenation of olefins and acetylenes and their derivatives," Journal of the Chemical Society A, 1966, pages 1711 to 1732, with a 1967 follow-up in the same journal extending the substrate-scope data. It became the first well-defined homogeneous catalyst shown to hydrogenate alkenes at rates rivaling the best heterogeneous catalysts of its era, establishing that a soluble, structurally characterized metal complex could carry out industrially relevant catalysis in solution, a landmark also noted by the University of Bristol's Molecule of the Month and the American Chemical Society's Molecule of the Week.

Frequently Asked Questions#

Is the Wilkinson catalyst still used today? Yes. Wilkinson's catalyst is still used for laboratory- and fine-chemical-scale hydrogenation of simple alkenes at mild conditions, 25°C and 1 atm H2, commonly at 1 to 5 mol% rhodium loading. Chemists reach for Crabtree's catalyst instead when the substrate is a tetrasubstituted or otherwise hindered alkene.

What is the IUPAC name for Wilkinson's catalyst? PubChem's systematic name for CID 84599 is "rhodium;tris(triphenylphosphane);chloride." Chemists more commonly call it chlorotris(triphenylphosphine)rhodium(I).

What is the geometry of Wilkinson's catalyst? A slightly distorted square-planar geometry around a four-coordinate, d8, 16-electron rhodium(I) center, confirmed by X-ray crystallography in both of the compound's orange and red crystalline forms.

What metal is present in Wilkinson's catalyst? Rhodium, platinum group metal element 45, in the rhodium(I) oxidation state, making up 11.1% of the compound's mass.

How is Wilkinson's catalyst prepared? By refluxing rhodium(III) chloride trihydrate with excess triphenylphosphine in ethanol. Triphenylphosphine supplies the ligands and reduces rhodium from the +3 to the +1 oxidation state, in a stoichiometrically necessary excess.

---