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Q5.22:
Discuss the nature of bonding in metal carbonyls.

Solution :

1. Fundamental Nature of Metal Carbonyls

Metal carbonyls are coordination complexes where carbon monoxide ($\text{CO}$) acts as a neutral ligand bonded to a transition metal in a low oxidation state (often zero). The bonding in these compounds is unique because it involves a synergistic effect known as synergic bonding, which strengthens the metal-carbon bond while simultaneously weakening the carbon-oxygen bond.

2. The Two-Fold Bonding Mechanism

The bonding arises from two simultaneous orbital interactions:

  • $\sigma$-Donation (Metal-Carbon $\sigma$-bond): The lone pair of electrons on the carbon atom of the $\text{CO}$ molecule is donated into an empty hybrid orbital of the transition metal atom. This forms a $\text{M} \leftarrow \text{CO}$ $\sigma$-bond.
  • $\pi$-Back-bonding (Metal-Carbon $\pi$-bond): The filled metal $d$-orbitals overlap with the empty antibonding $\pi^*$ molecular orbitals of the $\text{CO}$ ligand. This involves the donation of electron density from the metal to the ligand, forming a $\text{M} \rightarrow \text{CO}$ $\pi$-bond.

3. Visual Representation of Synergic Bonding

M C ≡ O σ-donation π-back-bonding

4. Consequences of the Synergic Effect

Interaction Electronic Effect Structural Consequence
$\sigma$-donation Electron density flows M $\leftarrow$ C Increases M-C bond order
$\pi$-back-donation Electron density flows M $\rightarrow$ C Decreases C-O bond order; Increases M-C bond order

Per the Molecular Orbital Theory, the back-donation of electron density into the $\pi^*$ orbital of $\text{CO}$ results in the reduction of the $\text{C} \equiv \text{O}$ bond order (from 3 towards 2). Consequently, the bond length of $\text{C}-\text{O}$ increases compared to free $\text{CO}$, while the $\text{M}-\text{C}$ bond length decreases, demonstrating partial double-bond character [In accordance with the Dewar-Chatt-Duncanson model].

5. Evidence and Summary

The extent of this bonding can be experimentally verified using Infrared (IR) Spectroscopy. As the metal-to-ligand $\pi$-back-donation increases, the $\text{C}-\text{O}$ stretching frequency decreases. This confirms that the strength of the metal-carbon bond is inversely proportional to the strength of the carbon-oxygen bond.

Final Solution: Bonding in metal carbonyls is characterized by a synergistic interaction consisting of $\sigma$-donation from the $\text{CO}$ ligand to the metal and $\pi$-back-donation from the metal to the empty antibonding $\pi^*$ orbitals of the $\text{CO}$ ligand. This stabilizes the complex through back-bonding, creating a shorter, stronger $\text{M}-\text{C}$ bond and a longer, weaker $\text{C}-\text{O}$ bond.


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