Catalysis by Organometallic complexes

As discussed, for heterogeneous catalysts the reaction starts with the adsorption of reactant on a vacant site on the catalyst surface. Similarly, for homogeneous organometallic complex catalysts, the reaction begins with the reactant molecule getting attached to a metal centre in the catalyst. Hence, the metallic centers must have vacant coordination sites. However, it is difficult to maintain vacant sites on the metallic center as the molecules are always in solvated conditions. Over all, the mechanism can be described as removal of ligand from the metallic center and addition of reactant to the vacant site. This mechanism is similar to substitution reaction. There is always competition for vacant sites between ligand, reactant or solvent molecules. The process is proposed to occur by associative or dissociative mechanism.

1. Dissociative mechanism

   In dissociative ligand exchange mechanism, initially there is breaking of bond between the metal and leaving ligand. This is the slowest and rate controlling step. A solvent molecule occupies the open site. Subsequently, the solvent is replaced by reactant in a fast step.

   

2. Associative mechanism
   
   In associative ligand exchange process, bond breaking of ligand from metal and bond formation between metal and reactant occurs simultaneously as shown below.

Elementary steps

The basic elementary steps that are involved in organometallic chemistry are discussed briefly.

1. Insertion
   
   In insertion mechanism an unsaturated molecule inserts into a metal- anion bond. In the shown example, the CO inserts into the metal (Pt)-methyl bond and acyl bond formed (R-CO) takes position of methyl group. The vacant position on metal is filled in solvent (S) molecule
   
   

   

2. Migration
   
   

   In migration mechanism, atom or group of atoms migrates from metal to hydrocarbon. In the shown example, hydride migrates from metal atom to ethene. The vacant position on metal is filled in by ligand (L) molecule.

   

3. ß-elimination
   
   

   This reaction is reverse of migration. The ß-elimination requires a vacant site at the complex.