Question

In $Fe{\left( {CO} \right)_5}$, the $Fe - C$ bond possesses:
A. $\pi -$character only.
B. both $\sigma$ and $\pi -$characters.
C. ionic character.
D. $\sigma -$character only.

Answer 1

To solve this question, we have to remember that the bond formed by the linear or axial or end to end overlapping of orbitals is called $\sigma$ (sigma) bond. The bond formed by the parallel or sidewise overlapping of orbitals is called the $\pi$ (pi) bond.

Complete step by step answer:
carbon has a unique property that it can form both the types of covalent bonds, i.e. the $\sigma$ and the $\pi$ bonds. A single bond contains only one $\sigma$ bond. Whereas, there are one $\sigma$ and one $\pi$ bond in case of a double bond and one $\sigma$ and two $\pi$ bonds in case of a triple bond.
In $Fe{\left( {CO} \right)_5}$, the $Fe - C$ bond is called synergic bonding. The ligand (CO) donates its lone pair of electrons to the vacant orbitals of the iron atom and forms the sigma-bond. Since the iron atom also possesses some electrons in its d-orbitals, it back donates those electrons to the molecular orbitals of the ligand forming a π-bond. The length of the metal-carbon bond is reduced in this manner and the complex gets more stability. One important thing to keep in mind is that the metal atom donates its electron pairs to CO's antibonding MO, so this synergistic bond weakens the C-O bond, resulting in a greater C-O bond length in the complex (as opposed to a free molecule of CO).
The length of metal-C bonds decreases, the length of C-O bonds increases, and stability becomes complex.
Hence, we can say that, In $Fe{\left( {CO} \right)_5}$, the $Fe - C$ bond possesses both $\sigma$ and $\pi -$characters.
So, the correct answer is “Option B”.

Note: CO is a ligand called carbonyl ligand. Actually, it is a dative bond and the ligand is attached to the central atom via a dative bond. The bond occurs between the lone pairs of ligands and the empty shells of the central atom. Here in case of $Fe{\left( {CO} \right)_5}$ the lone pair on carbon forms a dative bond with the iron.