Discovered by Sir Geoffrey Wilkinson, Wilkinson's catalyst is an inorganic
complex used to catalyse the
hydrogenation of
alkenes to alkanes. Its mechanism is well studied and fairly versatile; it is used mainly at laboratory scale.
It is a rhodium complex with the formula [RhCl(PPh3)3], that is, rhodium surrounded by one chloride ligand and three triphenylphosphine ligands. This is a typical example of dative bonding in a transition metal complex.
Its use in hydrogenation is as follows. Before activation, it is a square planar complex in which rhodium is in oxidation state 1. Dihydrogen, H2, homolytically dissociates into two hydrogen atoms which each coordinate to the rhodium in what is called oxidative addition: the metal's oxidation number increases to 3. The resulting octahedral complex then loses one of its phosphine ligands, which is replaced by the alkene.
One of the hydrogen atoms then transfers from the rhodium to the alkene, which becomes an alkyl ligand. The expelled phosphine ligand resumes its place in the complex and the remaining hydrogen then also transfers to the alkyl ligand, which becomes an alkane and leaves the complex. Thus the original Wilkinson complex, with all three phosphine ligands, is restored.
This reaction can be carried out at approximately atmospheric pressure and is applicable to a wide variety of alkenes. For example, 1-pentene can be hydrogenated to pentane. However ethene, and large, sterically hindered alkenes, cannot be used.