Question

Which of the following has lowest C-O bond order ?

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

Answer 1

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

The compounds with metal-carbonyl bonds are known as organometallic compounds. Here, ligands donate electron pair to the central metal atom and simultaneously accept electron pair from the central metal atom in its antibonding orbital of the C in $CO$.

Among the four options, option c has the central metal atom with the maximum tendency to donate electron pair with $Fe$ in ($-2$) with $CO$ as a ligand.

As the tendency of electron donation increases, synergic bonding between $M-C$ increases, thus increasing the number of electrons in the antibonding molecular orbital in $CO$. This decreases the $C-O$ bond order.

Bond order= $\frac{1}{2}${$N_b-N_a$]
Here Nb and Na represent a number of electrons in bonding and anti-bonding molecular orbitals. Therefore, c has the lowest $C-O$ bond order.