Question

Which of the following has longest C-O bond length? (Free C-O bond length in CO is 1.128 A.)

• A. $\left[Co\right(CO{)}_4{]}^{-}$

• B. $\left[Fe\right(CO{)}_4{]}^{2-}$

• C. $\left[Mn\right(CO{)}_6{]}^{+}$

• D. $Ni(CO{)}_4$

Answer 1

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

As negative charge on metal carbonyl complex increases, back $\pi$ -bonding increases and hence bond length of C-O bond increases while bond length of metal-carbon bond decreases.
Hence, $\left[Fe\right(CO{)}_4{]}^{2-}$ has longest C-O bond length among the given complexes.
The correct order of bond length of the given complexes is
$\left[Mn\right(CO{)}_6{]}^{+}<\left[Ni\right(CO{)}_4]<[Co(CO{)}_4{]}^{-}<[Fe(CO{)}_4{]}^{2-}$