Question

Which of the following has the longest metal-carbon $(M-C)$ bond length?

• A. $Ni(CO{)}_4$
• B. $\left[Co\right(CO{)}_4{]}^{-}$
• C. $\left[Mn\right(CO{)}_6{]}^{+}$
• D. $\left[Fe\right(CO{)}_4{]}^{2-}$

Answer 1

The correct option is C $\left[Mn\right(CO{)}_6{]}^{+}$

We know,
More the extent of back bonding, higher will be the bond order of $M-C$ bond and lower will be the bond order of $C-O$.

$\text{Bond length}\propto \frac{1}{\text{Bond order}}$

Here, $Mn$ has highest oxidation number so it has least tendency to donate electrons to the carbonyl compare to others.
Hence, its $M-C$ bond order is lowest and so it has longest bond length.

Hence, option (b) is correct.