Identify the complex and bonding type

The complex is $Fe(CO)_5$, i.e. iron pentacarbonyl.

In metal carbonyls, the bond between metal and carbon of CO is explained by synergic bonding:

• $\sigma$-donation from the lone pair on carbon of CO to the metal
• $\pi$-back donation from filled metal $d$-orbitals to the empty antibonding $\pi^*$ orbitals of CO

Nature of the Fe–C bond

Therefore, the $Fe-C$ bond contains:

• a $\sigma$ component due to donation $CO \to Fe$
• a $\pi$ component due to back-bonding $Fe \to CO$

So the bond is not only $\sigma$, not only $\pi$, and not primarily ionic.

Check the options

• A: $\pi$-character only $\rightarrow$ Incorrect
• B: both $\sigma$ and $\pi$ characters $\rightarrow$ Correct
• C: ionic character $\rightarrow$ Incorrect
• D: $\sigma$-character $\rightarrow$ Incomplete/incorrect, because $\pi$-back bonding is also present

Final answer

The $Fe-C$ bond in $Fe(CO)_5$ possesses both $\sigma$ and $\pi$ characters.