Question

Which of the following has highest C-O bond length ?

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

Answer 1

The correct option is C $\left[Fe\right(CO{)}_4{]}^{2-}$

$\left[Fe\right(CO{)}_4{]}^{2-}$ has highest C-O bond length
The central metal ions in the complexes $\left[Ni\right(CO{)}_4]$, $[CoC{O}_4{]}^{\Theta }$, $\left[Fe\right(CO{)}_4{]}^{2-}$ and $\left[Mn\right(CO{)}_6{]}^{+}]$ have the charge 0,-1,-2 and +1 respectively.
Thus iron in $\left[Fe\right(CO{)}_4{]}^{2-}$ has most negative charge. Hence, Metal to ligand backdonation is maximum in $\left[Fe\right(CO{)}_4{]}^{2-}$.
This electron density is in the antibonding MO of CO.
This results in lowest bond order and maximum bond length for C-O bond in $\left[Fe\right(CO{)}_4{]}^{2-}$.