Discuss the nature of Bonding in metal carbonyls.

Metal carbonyls are coordination complexes formed by the bonding of metal atoms with carbon monoxide (CO) ligands. The bonding in metal carbonyls involves several steps, and the nature of bonding can be explained as follows:

  1. Formation of the σ-bond (sigma bond):
  2. Carbon monoxide donates a lone pair of electrons from the carbon atom to the empty d-orbital of the metal.

  3. The overlap between the filled orbital of carbon and the empty orbital of the metal results in the formation of a sigma (σ) bond.

  4. Back Donation (π-donation):

  5. The metal, particularly transition metals, has d-orbitals available for π-bonding.
  6. The metal donates electron density back to the antibonding π*-orbital of carbon monoxide.

  7. This process is known as back donation or π-donation.

  8. Formation of π-bond (pi bond):

  9. The overlap between the filled π-orbital of carbon monoxide and the empty π*-orbital of the metal leads to the formation of a π-bond.

  10. Multiple Bonding:

  11. Metal carbonyls often exhibit multiple bonding, including σ-bonds and π-bonds.

  12. The presence of multiple bonds contributes to the stability and unique properties of metal carbonyls.

  13. Synergic Effect:

  14. The combination of σ-donation and π-back donation results in a synergic effect.

  15. This synergic effect enhances the stability of the metal carbonyl complex.

  16. Dewar-Chatt-Duncanson Model:

  17. The Dewar-Chatt-Duncanson model provides a comprehensive explanation of metal carbonyl bonding.

  18. It involves the interaction of metal d-orbitals with the σ-donating and π-accepting orbitals of carbon monoxide.

  19. Electron Counting:

  20. Understanding the electron count in metal carbonyls is crucial for predicting their stability and reactivity.
  21. The 18-electron rule is often applied, where the sum of metal d-electrons and the electrons donated by CO ligands is close to 18.

In summary, the bonding in metal carbonyls is a complex interplay of σ-donation, π-back donation, and the synergic effect, leading to the formation of multiple bonds and stability in these coordination complexes. The Dewar-Chatt-Duncanson model and electron counting provide valuable tools for understanding and predicting the behavior of metal carbonyls.