Question

In Fe(CO)₅ , the Fe-C bond possesses

• A. $\pi -$ character only
• B. Ionic character
• C. $\sigma -$ character only
• D. Both $\sigma$ and $\pi$ characters

Answer 1

Answer: (D)

In a metal carbonyl, the metal-carbon bond possesses both $\sigma$ and $\pi$ characters. A $\sigma$ -bond between metal and carbon atom is formed when a vacant hybrid orbital of the metal atom overlaps with an orbital of C atom of carbon monoxide containing a lone pair of electrons .
Formation of $\pi$ -bond is caused when a filled orbital of the metal atom overlap with a vacant antibonding ${\pi }^{\star }$ orbital of C atom of CO. This overlap is aslo called back donation of electrons by metal atom to carbon.

(a) The formation of the metal $\leftarrow$ carbon $\sigma$ - bond using an unshared pair of the C atom . (b) The formation of the metal $\to$ carbon $\pi$ - bond.
The $\pi -$ $\sigma$ -bond. overlap is perpendicular to the nodal plane of