Question

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

• A. $\pi$ - character only
• B. both $\sigma$ and $\pi$ - characters
• C. ionic character
• D. $\sigma$ - character only

Answer 1

The correct option is B both $\sigma$ and $\pi$ - characters

It's called “synergic bonding”. The ligand (CO) donates it's 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 it's d-orbitals, it back donates those electrons to the molecular orbitals of the ligand forming a π-bond. In this way, the metal-carbon bond length is reduced and the complex gets more stability. One important thing to keep in mind is that the metal atom donates it's electron pairs to the antibonding MO of CO, so the C-O bond is weakened by this synergic bonding, leading to a larger C-O bond length in the complex (as opposed to a free CO molecule).

Metal-C bond length reduces, C-O bond length increases, complex gets stability.

Hence, option $B$ is correct.