Bonding in Metal Carbonyls

Full Notes

Metal carbonyls are a special class of coordination compounds where carbon monoxide (CO) acts as a ligand to transition metals.

• Carbon monoxide (CO) is a neutral ligand.
• It binds through the carbon atom (not oxygen), using its lone pair.

Nature of Bonding

The bonding in metal carbonyls involves both sigma (σ) and pi (π) bonding between CO and the metal:

σ-Donation

• The lone pair of electrons on carbon in CO is donated into a vacant orbital of the metal.
• This forms a σ-bond (coordinate bond) from the ligand to the metal.

π-Back Donation (π-Back Bonding)

• The metal has filled d-orbitals, which can overlap with the empty π antibonding orbitals of CO.
• This allows electron density to flow back from the metal to the ligand.
• The result is M → CO π-back bonding.

Synergic Effect

These two interactions occur simultaneously and reinforce each other:

• σ-donation strengthens the M–CO bond, increasing electron density on the metal.
• π-back bonding stabilises the complex, but also weakens the C–O bond in the ligand.

Experimental Evidence

• In free CO, the C–O bond length is shorter (stronger).
• In metal carbonyls, the C–O bond length is longer, indicating weaker bonding due to π-back donation.
• This is supported by IR spectroscopy: the C–O stretching frequency decreases in metal carbonyls compared to free CO.

Examples of Metal Carbonyls

• [Ni(CO)₄] – Tetrahedral
• Fe(CO)₅ – Trigonal bipyramidal
• Cr(CO)₆ – Octahedral

All these are neutral complexes.

Summary

• Metal carbonyl bonding features σ-donation from CO and π-back bonding from the metal into CO π* orbitals.
• Synergic bonding strengthens M–CO and weakens C–O which is evident from lowered IR ν(CO) values.
• Common examples include [Ni(CO)₄], Fe(CO)₅, and Cr(CO)₆ with tetrahedral, trigonal bipyramidal, and octahedral geometries.