Question

Select the true statement from the following for metal carbonyls?

• A. $\pi$ back bonding strengthens $M-C$ bond order as well as $CO$ bond order
• B. $\pi$ back bonding weakens $M-C$ bond order as well as $CO$ bond order
• C. $\pi$ back bonding weakens $M-C$ bond order but strengthens $CO$ bond order
• D. $\pi$ back bonding strengthens $M-C$ bond order and weakens $CO$ bond order

Answer 1

Answer: (D)

The overlap of a lone pair on the $C$ atom with the empty hybrid metal orbital forms a metal-to-carbon $\sigma$-bond. The transition metal atom in a metal carbonyl has filled non-bonding $d$-orbitals which are of proper symmetry to overlap with the anti-bonding orbitals of $CO$. The electronic charge is transferred from the filled non-bonding orbitals of the metals to ${\pi }^{\ast }$ orbitals of the ligand $CO$. This reduces the bond order of $CO$. The $\pi$ back-bonding strengthens the $M-C$ bond order, it weakens the $C-O$ bond order.