Question

In metal carbonyls, the increase in C-O bond length in CO as compared to carbon monoxide is due to:

• A. The donation of lone pair of electrons on the carbon into a vacant orbital of the metal atom.
• B. The donation of a pair of electrons from a filled d-orbital of metal into the vacant antibonding ${\pi }^{\ast }$ orbital of carbon monoxide.
• C. (A) and (B) both
• D. None

Answer 1

The correct option is B The donation of a pair of electrons from a filled d-orbital of metal into the vacant antibonding ${\pi }^{\ast }$ orbital of carbon monoxide.

Donation of electrons from a filled d-orbital of metal into the vacant antibonding ${\pi }^{\ast }$ bonding orbital of CO decreases the bond order three to two and half or slightly more thus increasing the bond length between C-O.
Bond order $\alpha \frac{1}{\text{Bond length}}$