What is the nature of the synergic bonding mechanism in transition metal carbonyl complexes such as [Ni(CO)4] and [Fe(CO)5]?

Answer and explanation

Correct answer: A. σ-donation from carbon into metal d-orbital and π-backbonding from metal d-orbital into CO π* orbital

  1. Step 1: Metal carbonyls exhibit synergic (cooperative) bonding between the metal atom and carbon monoxide.
  2. Step 2: The carbon atom of donates its non-bonding sp lone pair into a vacant metal hybrid orbital ( or ), forming a coordinate -bond ().
  3. Step 3: Simultaneously, filled -orbitals of the transition metal overlap with vacant antibonding molecular orbitals of , back-donating electron density from metal to ligand ().
  4. Step 4: This -backbonding strengthens the bond while weakening the bond, creating a mutually reinforcing synergic effect.
Answer
A σ-bond is formed by donation of a carbon lone pair into a vacant metal d-orbital, while a π-bond is formed by back-donation from a filled metal d-orbital into vacant π* antibonding molecular orbitals of CO
Why not the others: 'A σ-bond is formed by transfer of electrons from oxygen...' is false because bonding occurs through carbon, which has the highest-energy HOMO; 'Pure electrostatic ionic attraction...' ignores the strong covalent orbital overlap; 'Formation of covalent bonds through complete transfer of electrons to oxygen...' is chemically inaccurate.

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