Which of the following has the longest $\mathrm{C}-\mathrm{O}$ bond length? (Free $\mathrm{C}-\mathrm{O}$ bond length in $\mathrm{CO}$ is $\mathrm{1}.128 \stackrel{\mathrm{o}}{\mathrm{A}}$).

Metal carbon bond in metal carbonyls possess both $\mathrm{\sigma }$ and $\mathrm{\pi }$ character. $\text{M}-\text{C}$ $\mathrm{\pi }$ bond is formed by donation of a pair of electrons from filled orbital of metal into vacant antibonding $\mathrm{\pi }$ orbital of $\text{CO}$ . $\text{CO}$ bond length increases if M has more tendency to donate lone pair.

Tendency to donate lone pair is ${\mathrm{Fe}}^{2-}>{\mathrm{Co}}^{-}>\mathrm{Ni}>{\mathrm{Mn}}^{+}$