Problem 165 - A Complete Resource Book in Chemistry for JEE Main

Question

In \(\mathrm{Fe}(\mathrm{CO})_{5}\), the Fe-C bond possesses (a) \(\pi\)-character only (b) both \(\sigma\) and \(\pi\) characters (c) ionic character (d) \(\sigma\) character only

Step-by-Step Solution

Answer

(b) both \(\sigma\) and \(\pi\) characters.

Step 1: Understanding the question

We need to determine the type of bonding character that exists between the Iron (Fe) atom and the Carbon monoxide (CO) ligands in the compound \( ext{Fe(CO)}_5\).

Step 2: Analyzing the bonding in \( ext{Fe(CO)}_5\)

The compound \( ext{Fe(CO)}_5\) is an example of a metal carbonyl. In such complexes, carbon monoxide acts as a ligand, bonding to the metal through a combination of a \(\sigma\)-bond and a \(\pi\)-back bonding mechanism.

Step 3: Understanding \(\sigma\)-bonding

The \(\sigma\)-bond is formed by the donation of a lone pair of electrons from the carbon atom (which acts as a donor) in CO to the vacant orbitals of the iron atom. This forms a strong sigma bond between the iron and carbon atoms.

Step 4: Understanding \(\pi\)-bonding

\(\pi\)-bonding, or \(\pi\)-back bonding, occurs when the metal donates electrons back into the anti-bonding \(\pi^*\) orbitals of the CO ligands. This dual bonding interaction strengthens the metal-carbon bonding.

Step 5: Concluding the bonding character

Given both \(\sigma\)-bonding and \(\pi\)-bonding are present, the Fe-C bond in \(\text{Fe(CO)}_5\) possesses both \(\sigma\) and \(\pi\) characters.

Key Concepts

Sigma Bonding

Sigma bonding is essential to understand within the context of metal carbonyl complexes like \( \text{Fe(CO)}_5 \). This type of bond forms when an electron pair is donated from the carbon monoxide ligand directly to a metal center—in this case, iron. The carbon monoxide molecule has a lone pair of electrons located on the carbon atom. These electrons form a strong, direct, and end-to-end overlap with a vacant orbital on the iron atom.
This direct overlap creates a "head-on" or linear type of bond that is both stable and robust. Sigma bonds are crucial as they form the primary connection that holds the ligand and metal together in these complexes. With healthy electron involvement, sigma bonding offers

• stability to the metal-ligand complex,
• strength by sharing electron density, and
• foundation for further bonding interactions, like pi bonding.

Thus, sigma bonding plays a key role in defining the structure and properties of metal carbonyls.

Pi Bonding

Pi bonding, or pi back bonding, is a fascinating phenomenon that enhances the stability of metal carbonyl complexes. After forming a sigma bond, metals can further interact with the carbon monoxide ligands through pi bonding. For pi bonding to occur, the metal must be able to donate electron density into the unoccupied anti-bonding \(\pi^*\) orbitals of carbon monoxide.
This interaction is typically illustrated as a back-donation from the metal to the ligand because the metal center supplies electron density to the ligand. The result is a mutually beneficial exchange where

• the ligand stabilizes the metal,
• strengthens the bond between ligand and metal, and
• influences the electronic properties of the entire complex.

Such pi bonding introduces partial double bond character between the metal and carbon, thus strengthening the overall connection. In compounds like \( \text{Fe(CO)}_5 \), this dual bonding interaction is essential for understanding its chemical nature.

Back Bonding

Back bonding, a crucial aspect of pi bonding, occurs in metal carbonyl compounds when the metal center engages in electron exchange with the ligand. Specifically, back bonding happens when metal atoms possess enough electron density to share with the carbon monoxide ligands via their anti-bonding orbitals. This transfers electrons from a filled d orbital on the metal to the empty \(\pi^*\) orbital of CO.
This interaction is vital for the stability of the complex. When effective, back bonding leads to:

• increased electron sharing between metal and ligand,
• reduction of electron deficiency of the metal, and
• reinforcement of the existing sigma bond.

These effects make back bonding an influential factor in determining the structural and electronic properties of complexes like \( \text{Fe(CO)}_5 \). Ultimately, the back bonding reinforces the dual nature of bonding interactions that comprise both sigma and pi components, showcasing the intricate chemistry of metal-ligand bonds.