Wilkinson's catalyst, named after the chemist Sir Geoffrey Wilkinson, is represented by the complex [Ph3P)3RhCl]. It is an important catalyst used in various organic reactions, particularly in hydrogenation reactions.

Structure:
- The complex contains a central rhodium atom (Rh) coordinated with three triphenylphosphine ligands (Ph3P) and one chloride ion (Cl-).
- The phosphine ligands are tridentate ligands, meaning they coordinate to the central metal atom through three electron pairs.
- The chloride ion acts as a monodentate ligand, coordinating to the central metal atom through a single electron pair.

Catalytic Activity:
- Wilkinson's catalyst is widely used in hydrogenation reactions, where it facilitates the addition of hydrogen (H2) to unsaturated compounds.
- The phosphine ligands stabilize the catalyst, making it more efficient in catalyzing the reaction.
- The chloride ion helps in the activation of the hydrogen molecule by coordinating with the rhodium atom.

Mechanism:
1. The unsaturated substrate approaches the catalyst, and one of the phosphine ligands moves away to allow coordination of the substrate to the rhodium center.
2. The substrate binds to the rhodium center, forming a complex.
3. Hydrogen molecules approach the complex, and the chloride ion helps in the activation of the hydrogen molecule by coordinating with the rhodium atom.
4. The coordinated hydrogen molecule undergoes oxidative addition, breaking the H-H bond and forming a dihydride complex.
5. The substrate is then reduced by the dihydride complex, leading to the hydrogenation of the substrate.
6. After the reaction, the product is released, and the catalyst is regenerated by the addition of another phosphine ligand.

Applications:
- Wilkinson's catalyst is used in various hydrogenation reactions, including the hydrogenation of alkenes, alkynes, and carbonyl compounds.
- It is also used in the synthesis of pharmaceuticals, fine chemicals, and polymers.

In summary, Wilkinson's catalyst is a rhodium-based complex that is widely used as a catalyst in hydrogenation reactions. Its structure, catalytic activity, mechanism, and applications make it an important tool in organic synthesis.