In $$Fe(CO)_5$$, the $$Fe - C$$ bond possesses

The ligand CO contains a lone pair on the carbon atom in the $$3\sigma$$ molecular orbital and also has empty $$\pi^{*}$$ antibonding orbitals that can accept electron density.

When CO binds to a transition-metal centre such as $$Fe$$, two simultaneous interactions occur:

1. $$\sigma$$-donation (forward donation):
The carbon lone pair donates electron density to an empty orbital of the metal. This forms a metal-ligand $$\sigma$$ bond.

2. $$\pi$$-back donation (back bonding or synergic bonding):
Filled $$d_{\pi}$$ orbitals of the metal overlap with the empty $$\pi^{*}$$ orbitals of CO and donate electron density back to the ligand. This generates a metal-ligand $$\pi$$ bond oriented perpendicular to the $$\sigma$$ bond.

Because both interactions operate simultaneously, every $$Fe - C$$ linkage in $$Fe(CO)_5$$ contains:

• one $$\sigma$$ component due to lone-pair donation from CO to Fe, and
• one $$\pi$$ component due to back donation from Fe to CO.

Hence the $$Fe - C$$ bond possesses both $$\sigma$$ and $$\pi$$ characters.

Option B which is: both $$\sigma$$ and $$\pi$$ characters