Question

Which is not true about metal carbonyls?

• A. Here $d\pi -p\pi$ back bonding takes place
• B. Here $p\pi -p\pi$ back bonding takes place
• C. Here metal acts as Lewis base as well as Lewis acid
• D. Here $CO$ acts as a Lewis base as well as Lewis acid

Answer 1

The correct option is B Here $p\pi -p\pi$ back bonding takes place

In metal carbonyls, both metal and CO acts as Lewis acid as well as base since both donate and accepts pair of electrons.

Carbon of $CO$ donates electron pair to the empty d-orbital of metal and in return, metal donates electron pair to ${\pi }^{\ast }$ molecular orbital $CO$ which results in dπ−pπ back bonding, i.e., sideways overlap of a filled orbital of the metal with empty antibonding ${\pi }^{\ast }$ (LUMO) orbital of CO.

Hence, option (d) is the wrong statement.