Module B6
Organometallic Catalysis
Concept Overview
Organometallic compounds serve as powerful homogeneous catalysts. Their catalytic power relies on the metal cycling through key reaction steps:
- Ligand Substitution: Exchange of ligands (associative for 16e⁻, dissociative for 18e⁻).
- Oxidative Addition (OA): Metal inserts into a covalent bond. OS increases by +2, CN increases by +2. Requires ≤16e⁻ and electron-rich center.
- Reductive Elimination (RE): Reverse of OA — two cis ligands couple and depart. OS decreases by −2, CN decreases by −2.
- Migratory Insertion: An unsaturated ligand inserts into an adjacent M–R bond, generating a vacant site.
Wilkinson's Catalyst () is a 16e⁻ complex for homogeneous hydrogenation of alkenes.
Key Equations
| Reaction | ΔOS | ΔCN | Requirement |
|---|---|---|---|
| Oxidative Addition | +2 | +2 | ≤16e⁻, electron-rich metal |
| Reductive Elimination | −2 | −2 | Two cis ligands, electron-poor or sterically crowded |
| Migratory Insertion | 0 | −1 | Cis unsaturated ligand + M–R bond |
| Ligand Substitution | 0 | 0 | Labile ligand or open site |
Worked Examples
Identifying Reaction Steps
Classify:
Common Misconceptions
❌ Misconception
Oxidative addition can occur at any metal center.
✅ Correction
OA requires the metal to have a stable OS +2 higher, and space for two new ligands. 18e⁻ complexes must lose a ligand first.
Interactive Visual
Step through the Wilkinson catalytic cycle for alkene hydrogenation:
Step 1 of 6Substitution
Ligand Dissociation
PPh₃ dissociates from [RhCl(PPh₃)₃] to open a coordination site.
Rh OS
+1
VE Count
14e⁻
Formative Quiz
Question 1
What happens to the metal oxidation state during reductive elimination?