Which of the following carbonyls will have the strongest C – O bond

Options

(a) Mn(CO)₆⁺
(b) Cr(CO)₆
(c) V(CO)₆⁻
(d) Fe(CO)₅

Correct Answer:

Mn(CO)₆⁺

Explanation:

The presence of positive charge on the metal carbonyl would resist the flow of the metal electron charge to π*-orbitals of CO. This would increase the CO bond order and hence CO in a metal carbonyl cation would absorb at a higher frequency compared to its absorption in a neutral metal carbonyl.