Because transition metals and main-group elements operate under very different bonding and electronic rules, their reaction mechanisms also differ.
Key Differences in Reaction Mechanisms
1. Availability of d-Orbitals
- Transition metals: Have partially filled d-orbitals, which allow variable coordination numbers, oxidation states, and bonding modes.
→ This enables mechanisms like ligand exchange, oxidative addition, reductive elimination, and electron transfer. - Main-group elements: Typically use s and p orbitals, with limited ability to expand coordination numbers (except heavier elements using d-orbitals).
→ Their reactions follow more classical covalent bond mechanisms (substitution, addition, elimination).
2. Oxidation State Changes
- Transition metals: Can easily undergo redox changes during a mechanism.
Example: Pd(0) → Pd(II) in oxidative addition. - Main-group elements: Usually have fixed oxidation states dictated by the octet rule.
Example: Carbon in methane almost never changes oxidation state during substitution.
3. Reaction Pathways
- Transition metals:
- Ligand substitution → associative, dissociative, interchange.
- Electron transfer → inner-sphere, outer-sphere.
- Organometallic steps → oxidative addition, reductive elimination, migratory insertion, β-hydride elimination.
- Main-group elements:
- SN1 / SN2 substitution (at saturated centers).
- E1 / E2 elimination.
- Electrophilic or nucleophilic addition (to π-bonds).
- Radical mechanisms (in halogenation, polymerization).
4. Coordination Flexibility
- Transition metals: Can change coordination number during a mechanism (e.g., octahedral → 5-coordinate intermediate).
- Main-group elements: Usually constrained by octet rule (CN = 4 for C, CN = 3 for N, etc.), so mechanisms don’t involve large coordination changes.
5. Catalysis
- Transition metals: Widely used in homogeneous and heterogeneous catalysis because they can shuttle between oxidation states and accommodate multiple ligands.
→ Example: Wilkinson’s catalyst in hydrogenation. - Main-group elements: Participate in catalysis, but usually by acid-base chemistry or radical processes, not by redox cycling.
→ Example: AlCl₃ in Friedel–Crafts reactions.
In short:
- Transition metals: Flexible coordination, variable oxidation states, d-orbital involvement → mechanisms include ligand exchange, redox, and organometallic steps.
- Main-group elements: Governed by octet rule, fixed valence, and classical covalent bonding → mechanisms follow substitution, elimination, and addition pathways.