Indeed, Wilkinson’s catalyst is a pre-catalyst that is converted to an active form by losing one triphenylphosphine ligand before entering the catalytic cycle. Usually, the solvent molecule fills the vacant site.
Initially, the catalyst activates the molecular dihydrogen by oxidative addition mechanism to give a 18 valence electron dihydrido complex. The oxidation state of Rh is increased to +3. Thus formed dihydrido complex binds to the olefin in the next step with the concomitant loss of solvent or PPh3 ligand. Since the activation of dihydrogen occurs before addition of olefin, this path is referred to as dihydride path .
Now one of the hydrogen undergoes migratory insertion at the double bond. This is a slow step i.e., Rate Determining Step (RDS).
Immediately and finally, the alkane is released rapidly by an irreversible reductive elimination step that completes the catalytic cycle. The oxidation state of Rh is decreased to +1 and the catalyst is regenerated.
However, other paths and intermediates are also possible under the given reaction conditions (see link below).
When it is dissolved in a solvent like benzene-ethanol, one of the phosphine molecules can be replaced by a weakly-bound solvent molecule, giving what is effectively a three-coordinate rhodium complex. Under an atmosphere of hydrogen gas, the resulting complex adds a hydrogen molecule, breaking the H-H bond forming a five-coordinate dihydride complex of rhodium. The dihydride is a Rh(III) species, so this is an oxidative-addition reaction, during which the colour of the solution changes from red to yellow. [RhH2Cl(PPh3)2] is still a 16-electron species and also has a vacant coordination site, so it can add an alkene molecule, forming a six-coordinate 18-electron complex. Next, there is a rearrangement with the coordinated alkene being inserted into a rhodium-hydrogen bond to form an alkyl complex – alternatively, think of it as a hydride transfer to the coordinated alkene. This step is rapidly followed by the transfer of the other hydrogen from rhodium to the alkyl group. This generates an alkane, which is immediately lost in a reductive-elimination step, so that the catalytic cycle – shown below in simplified form – can begin again.