Wilkinson Catalyst

Wilkinson’s catalyst is a coordinate compound of rhodium. It enables the reaction to proceed in a faster manner. It is used in the hydrogenation reaction of the unsaturated organic compound. In this article, we have covered the IUPAC name of the Wilkinson catalyst, its uses, its structure, and its mechanism.

Wilkinson Catalyst Formula

The Wilkinson catalyst formula is [RhCl(PPh3)3]. The Wilkinson Catalyst Formula represents the coordinate compound of rhodium. Its formula is made up of one rhodium, one chloride, and three PPh3 units.

IUPAC Name of Wilkinson Catalyst

The IUPAC name of the Wilkinson catalyst is Chloridotris (triphenylphosphine) rhodium(I)].

Wilkinson's Catalyst Structure

In Wilkinson’s catalyst structure the rhodium is bonded with three triphenylphosphine units by three single covalent bonds and one chlorine element is bonded with rhodium by a single covalent bond. Wilkinson catalyst hybridisation is dsp2. The dsp2 hybridisation represents the square planar shape. It is an inner complex molecule because the orbital involved in this compound is inner 3d.

Properties of Wilkinson Catalyst

Physical Properties of Wilkinson Catalyst

• The Wilkinson catalyst exists in the solid-state.

• The Wilkinson catalyst is red-brown.

• The Wilkinson catalyst is not soluble in polar compounds such as water and it is soluble in nonpolar compounds such as benzene, dichloromethane, and tetrahydrofuran.

• The molecular mass of the Wilkinson catalyst is 925.22 grams/mole.

• The melting point of the Wilkinson catalyst is around 520 K.

Chemical Properties of Wilkinson Catalyst

The chemical properties shown by the Wilkinson catalyst are the result of the electronic configuration and the shape attained by the catalyst.

• The hybridisation of the Wilkinson catalyst is dsp2 and its shape is a square planner.

• Wilkinson catalyst RhCl(PP3)3 reacts with the carbon monoxide and forms bis (triphenylphosphine) rhodium carbonyl chloride RhCl(CO)(PPh3)2 as a product. This formed product compound can also be formed by the reaction with the aldehydes.

RhCl(PPh3)3 + RCHO → RhCl(CO)(PPh3)2 + RH + PPh3

• Wilkinson catalysts can form a dimer when mixed in the benzene solution. This formed dimer [RhCl(PPh3)3 is poorly soluble and red.

• The Wilkinson catalyst can be converted into the hydridotetrakis (triphenylphosphine) rhodium (I) HRh (PPh3)4 on reacting it with the alkali, hydrogen, and excess triphenylphosphine.

RhCl(PP3)3 + H2 + base → HRh (PPh3)4

Wilkinson Catalyst Uses

• The Wilkinson catalyst is widely used for the hydrogenation reaction of unsaturated hydrocarbons (olefins). It adds the molecular hydrogen at an unsaturated carbon position in the compound.

• The Wilkinson catalyst can be used in the addition of a hydrogen-acyl group to the alkenes.

• It plays a major role in the hydroboration reaction of the alkenes.

• The Wilkinson catalyst is used in the selective hydrogenation of the alkenes. It preferably adds the hydrogen at the least hindered unsaturated carbon position.

Catalytic Hydrogenation of Alkenes

The Wilkinson catalyst is widely used in the hydrogenation process. It adds molecular hydrogen to the unsaturated carbon in the compound. It preferably adds hydrogen to the less hindered carbon.

Catalytic Hydrogenation Mechanism

• The first step involved in the hydrogenation mechanism is the dissociation of the triphenylphosphine ligands, which results in the formation of a 14 or 12 electron complex.

• Then it undergoes oxidative hydrogenation.

• The pi complex with the alkene is formed.

• Migratory insertion and reductive elimination complete the alkane formation step.

Preparation of the Wilkinson Catalyst

Wilkinson catalysts can be prepared from the hydrated form of rhodium(III) chloride. Rhodium (III) chloride undergoes the reaction with excess triphenylphosphine with ethanol. Here, ethanol acts as a refluxing agent. Triphenylphosphine is a good reducing agent and is represented as P(C6H5)3. In the preparation reaction of the Wilkinson catalyst, triphenylphosphine reacts with the rhodium chloride and reduces it from the +3 oxidation state to the +1 oxidation state.

4P(C6H5)3 + RhCl3(H2O)3 → RhCl(P(C6H5)3)3 + OP(C6H5)3 + 2HCl + 2H2O

Did You Know?

• Wilkinson catalyst represents the name of chemist and Nobel laureate Sir Geoffrey Wilkinson. He first popularized its use.

• The oxidation state of rhodium is +4 to -3. In the Wilkinson catalyst, the oxidation state of the rhodium is +1.

• Wilkinson catalysts have the ability to reduce double bonds or alkenes. It is a red-brown colored solid. The Wilkinson catalyst is soluble in hydrocarbon solvents such as dichloromethane tetrahydrofuran and Benzene. This is a widely used compound in the halogenation of alkenes.

Formula of Wilkinson Catalyst

The formula of the Wilkinson catalyst is RhCl(PPh₃)₃.

The scientific name that describes its structure as well is chlorotris(triphenylphosphine)rhodium(I) in which rhodium is +1 in an Oxidation State.

Structure

Rhodium in Wilkinson's catalyst has four complexes that allow Rhodium to reduce the double bond. to form a complex.

The large size of the molecule allows it to reduce the least hindered double bond. Wilkinson's catalyst will only reduce the least hindered double bond, even when there is more than one double bond on the molecule because that is where the catalyst is able to fit.

Properties

• Melting point - 518 - 523k

• Molar mass - 925.22gms/mole

• Soluble in - hydrocarbon solvents 

• It has a square planar coordination geometry.

• The compound when stirred into a solution of benzene undergoes dimerization.

• Yields (RhCl(CO)(PPh3)2) when it reacts with carbon monoxide.

Uses

• Extensively used for halogenation of alkenes.

• Selectively reduces double bonds or alkenes which makes it useful in the reduction of a specific double bond in a molecule.

Preparation

The catalyst can be obtained by treating an excess of triphenylphosphine in reflecting ethanol with rhodium(|||) chloride hydrate. Triphenylphosphine oxidizes itself from the oxidation state by serving as a two-electron reducing agent.