IIT-JEE 2007 · Paper 2, Chemistry question 24

Back-bonding. The more electron density on the metal, the more it donates into the CO π* orbitals, weakening the C-O bond.

$\ce{[Mn(CO)6]+}$, $\ce{[Cr(CO)6]}$ and $\ce{[V(CO)6]-}$ are isoelectronic (d⁶ metal centres). The anion $\ce{[V(CO)6]-}$ has the most electron-rich metal (V in the −1 state), so it back-bonds most and its C-O bond order is lowest; the cation $\ce{[Mn(CO)6]+}$ back-bonds least. Neutral $\ce{[Fe(CO)5]}$ lies in between.

(D)