Question

Which of the following has the longest $C-O$ bond length? (Free $C-O$ bond length in $CO$ is $1.128\overset{o}{A}$ ).

• A. $Ni\left(C O\right)_{4}$
• B. $\left[ Co ( CO )_{4}\right]^{-}$
• C. $\left[ Fe ( CO )_{4}\right]^{2-}$
• D. $\left[ Mn ( CO )_{6}\right]^{+}$

Answer 1

Answer: (C)

Metal carbon bond in metal carbonyls possess both $\sigma $ and $\pi $ character. $\text{M} - \text{C}$ $\pi $ bond is formed by donation of a pair of electrons from filled orbital of metal into vacant antibonding $\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 $Fe^{2 -}>Co^{-}>Ni>Mn^{+}$