Organometallic Reactions

Organometallic reactions involve transformations at metal-carbon bonds and play a central role in catalysis, organic synthesis, and industrial processes. These reactions are governed by the interplay of metal-ligand bonding, oxidation state changes, and coordination geometry. Below is a summary of key reaction types:

1. Ligand Substitution

  • Definition: Exchange of one ligand for another at a metal center.

  • Mechanisms:

    • *Associative (ASN2-like):* Incoming ligand binds first (common in 16e- complexes).

    • *Dissociative (D/SN1-like):* Leaving group departs first (typical for 18e- complexes).

  • Importance: Fundamental step in catalytic cycles.

2. Oxidative Addition

  • Definition: Cleavage of an X–Y bond (e.g., H–H, C–X) by a metal, increasing its oxidation state by +2.

  • Mechanisms:

    • Concerted (3-center transition state).

    • SN2-like (nucleophilic attack).

    • Radical pathways.

  • Applications: C–H activation, cross-coupling.

3. Reductive Elimination

  • Definition: Coupling of two ligands to form a new bond (X–Y), reducing the metal’s oxidation state by –2.

  • Key Points:

    • Requires cis arrangement of ligands.

    • Reverse of oxidative addition.

  • Role in Catalysis: Forms C–C/C–X bonds (e.g., in Pd-catalyzed couplings).

4. Migratory Insertion

  • Definition: Insertion of an unsaturated ligand (CO, alkene) into an adjacent M–X bond (X = H, alkyl).

  • Examples:

    • CO insertion → acyl complexes (hydroformylation).

    • Olefin insertion → chain growth (polymerization).

  • Stereochemistry: Retention at chiral centers.

5. β-Hydride Elimination

  • Definition: Elimination of a β-hydrogen from a metal alkyl to form a metal hydride and alkene.

  • Requirements:

    • Syn-coplanar M–Cα and Cβ–H bonds.

    • Vacant coordination site.

  • Applications: Alkene formation, catalyst regeneration.

6. Nucleophilic Attack on Ligands

  • Concept: Metal coordination activates ligands (e.g., CO, alkenes) toward nucleophiles.

  • Examples:

    • Aldehyde synthesis from metal-CO complexes.

    • Wacker oxidation (nucleophilic attack on coordinated ethylene).