Question

Among the following metal carbonyls, the $C-O$ bond order is lowest in

• A. $[Mn(CO)_6]^+$
• B. $[Fe(CO)_5]$
• C. $[Cr(CO)_6]$
• D. $[V(CO)_6]^-$

Answer 1

Answer: (B)

$[Fe(CO)_5]$

Answer 2

Greater the extent of d$\pi -p \pi$ back bonding, smaller will be the bond order of CO bond in metal carbonyls. In $Fe(CO)_5 ,$ there is maximum number of valence shell electrons (d-electrons), greatest chances of $p \pi - d \pi$ back bonding, lowest bond order of CO bond.