JEE Advanced · 2007 · 08 Apr 2007 · Paper 2 · Coordination Compounds

The C-O bond order is lowest in the carbonyl with maximum back-bonding from metal d-orbitals to CO π* orbitals. Fe(0) in $\left[\mathrm{Fe}(\mathrm{CO}{)}_5\right]$ has electronic configuration $3d^8$, providing four lone pairs for back-bonding with CO. This is more than Mn⁺ ($3d^6$, three lone pairs), Cr⁰ ($3d^6$, three lone pairs), or V⁻ ($3d^6$, three lone pairs). Maximum back-bonding weakens the C-O bond the most, giving the lowest bond order.