Metal Carbonyl Compounds
Concept Overview
Metal carbonyl bonding involves the synergic bonding mechanism:
- σ-donation: The lone pair on CO donates into an empty metal d-orbital, forming a M–C σ-bond.
- π-backbonding: The electron-rich metal shifts density from filled d-orbitals into the empty π* anti-bonding orbital of CO.
Consequences of back-donation:
- Strengthens the M–C bond (shorter, stronger).
- Weakens the C≡O bond (lower stretching frequency in IR).
Free CO has . Coordinated CO shifts to 1800–2000 cm⁻¹. Anionic complexes push it even lower due to enhanced back-bonding.
Key Equations
Key relationship: More electron density at the metal → more π-backbonding → weaker C–O bond → lower .
| Bonding Mode | ν(CO) Range | Description |
|---|---|---|
| Terminal | 1850–2120 cm⁻¹ | Single M–C≡O bond. Most common mode. |
| μ₂-Bridging | 1750–1850 cm⁻¹ | CO bridges two metal centers. More back-bonding weakens C–O. |
| μ₃-Bridging | 1620–1730 cm⁻¹ | CO bridges three metal centers. Maximum back-bonding. |
Worked Examples
Use the interactive IR graph below to see how complex charge affects the C–O stretching frequency. Drag the slider to explore the relationship between back-bonding and ν(CO).
Common Misconceptions
❌ Misconception
A lower ν(CO) means the M–C bond is weaker.
✅ Correction
Lower ν(CO) means the C≡O bond is weaker (more π* occupation). The M–C bond is actually stronger due to enhanced back-bonding.
Interactive Visual
Adjust the complex charge to see how ν(CO) shifts with back-bonding intensity: