Lecture Notes: Metal Carbonyls and Nature of Bonding in Metal Carbonyls

Key Concepts

• Metal Carbonyls: Transition metal complexes with CO (carbon monoxide) as ligands.
• Homoleptic vs. Heteroleptic Complexes:
  • Homoleptic: All ligands are identical (e.g., Ni(CO)₄).
  • Heteroleptic: Different types of ligands.

Historical Context

• First Metal Carbonyl: Nickel tetracarbonyl (Ni(CO)₄) discovered by scientists.
• Importance: These compounds have significant catalytic properties.

Nature of Bonding in Metal Carbonyls

1. Sigma Bond Character:

   • Formation: Ligand (CO) donates a lone pair of electrons to the metal atom.
   • Representation: Often depicted in molecular orbital diagrams.
   • Mechanism: The lone pair from the filled orbital of CO is donated to the vacant d-orbital of the metal, forming a sigma bond.

2. Pi (π) Bond Character:

   • Back Bonding: Metal donates electrons back to the CO ligand.
   • Mechanism: Electron density is transferred from the filled d-orbital of the metal to the vacant π* (antibonding) orbital of CO.
   • Effect: Balances the electron density, strengthens the bond.*

Molecular Orbital Theory

• Electron Distribution: CO molecule has 14 electrons distributed in molecular orbitals.
  • Sigma and Pi Orbitals: Electrons fill bonding orbitals first, followed by antibonding orbitals.
  • Bond Formation: Initial sigma bond from lone pair donation, subsequent pi bond from back donation.

Synergic Effect

• Definition: Combined effect of sigma donation and pi back-donation.
• Strength: Results in a very strong metal-CO bond.
• Forward Bond (Sigma): Donation from ligand to metal.
• Backward Bond (Pi): Donation from metal to ligand's π* orbital.*

Important Points

• Electrons and Orbitals: Proper understanding of electron distribution in molecular orbitals is crucial.
• Bonding Characteristics: Both sigma and pi characteristics are important for the stability and reactivity of metal carbonyls.
• Applications: Catalytic properties and the role in organometallic chemistry.