Introduction:

Metal carbonyls are coordination compounds in which a transition metal is bonded to one or more carbonyl (CO) ligands. These compounds play a crucial role in organometallic chemistry due to their unique bonding characteristics, stability, and industrial applications. The bonding in metal carbonyls is explained using the synergic bonding model, involving both σ-donation and π-back donation interactions.



Bonding in Metal Carbonyls – Class 12 Chemistry:

Metal carbonyls exhibit fascinating bonding properties due to the presence of carbonyl ligands, which interact with the metal center through a combination of donation and back-donation mechanisms.

Structure and Composition of Metal Carbonyls:

- Metal carbonyls are composed of a transition metal center bonded to CO ligands.

- The general formula is M(CO)x, where ‘M’ represents the metal, and ‘x’ denotes the number of CO ligands.

- Examples: Ni(CO)4, Fe(CO)5, Cr(CO)6.

- These compounds can be mononuclear or polynuclear.

Read Also: Bonding in Coordination Compounds – Class 12 Chemistry Study Notes

Synergic Bonding in Metal Carbonyls:

The bonding in metal carbonyls follows a synergic interaction involving:

1. σ-Bonding: The carbonyl ligand donates electron density to the metal via the lone pair on carbon (M ← CO).

2. π-Back Bonding: The metal donates electron density from its d-orbitals into the empty π* (antibonding) orbitals of CO (M → CO).

Molecular Orbital Approach to Metal Carbonyl Bonding:

- The metal-carbon bond is a result of molecular orbital interactions.

- The carbonyl ligand acts as both a donor and acceptor, stabilizing the complex.

- The strength of back bonding depends on:

 - The oxidation state of the metal.

 - The number of CO ligands.

 - The nature of the metal (e.g., Fe, Ni, Cr).

Infrared Spectroscopy and Bonding in Metal Carbonyls:

- The C≡O stretching frequency (νCO) in metal carbonyls helps determine the extent of π-back bonding.

- Strong back bonding reduces the CO stretching frequency (lower νCO).

- Examples:

 - Free CO: ~2143 cm⁻¹

 - Fe(CO)5: ~2000 cm⁻¹ (due to back bonding)

Applications of Metal Carbonyls:

1. Catalysis:

  - Used in industrial catalysts (e.g., hydroformylation and carbonylation).

2. Organic Synthesis:

  - Metal carbonyls are precursors for many organometallic compounds.

3. Industrial Processing:

  - Nickel carbonyl [Ni(CO)4] is used in metal refining.

4. Medical Applications:

  - Some metal carbonyls are explored for drug development.

Conclusion:

Metal carbonyls are essential coordination compounds exhibiting unique synergic bonding through σ-donation and π-back bonding. Their structural properties influence their stability, spectroscopic behavior, and industrial applications. Understanding these interactions provides deeper insights into transition metal chemistry.