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 given compound is a metal carbonyl . Metal carbonyls are the organometallic compounds in which carbon monoxide $(CO)$ acts as the ligand . We have to find the bond which $Fe - C$ possess .

Complete step by step answer:
In metal carbonyl the formation of bonds between the metal and carbon atom is as follows :
The first overlap takes place between the filled bonding ${\pi _{2p}}$ orbital of the carbon monoxide with an empty metal d-orbital resulting in a $\sigma$ - bond between the metal and carbon atom of carbon monoxide. Here, donation of lone pair of electrons on carbon into a vacant d-orbital of the metal takes place . As $CO$ is a weak base/weak donor, the $\sigma$ - bond formed with the metal atom is weak.
The second overlap takes place between the filled metal d-orbital with an empty antibonding ${\pi _{2p}}$ -orbital of the carbon monoxide resulting in additional $\pi$ bond between the metal and same carbon monoxide molecule. Here, donation of electrons from a filled metal d - orbital into a vacant antibonding $\pi$ - orbital of $CO$ occurs (back bonding). Here, $CO$ is acting as an acceptor ligand.
The effect of $\sigma$ bond formation strengthens the $\pi$ bond and vice-versa. This is called synergic effect (that is , working towards the same goal) . Thus as a result of synergic effect, the bond between $CO$ and metal is strengthened.
Further , as explained above, a weak $\sigma$ bond is formed by donation of electron pair from carbon to metal $(M \leftarrow C \equiv O)$ and a stronger $\pi$ bond (second bond) is formed by back donation of electrons from filled d-orbitals of metal into empty antibonding ${\pi ^*}$ orbital is carbon $(M \to C\,bond)$ .
Hence , In $Fe{(CO)_5}$ , the $Fe - C$ bond possesses both $\sigma$ and $\pi$ characters .
So , option B is correct .

Note: Metal carbonyls like $Fe{(CO)_5}$ are volatile and toxic . Further , they are either colourless or they have a light colour . They are solid at room temperature and atmospheric pressure .