Question

The CO molecule has IR stretching frequency of 2143 cm-1 but it shifts to different regions in metal carbonyls explain​

Answer 1

The IR stretching frequency of CO molecule shifts to different regions in metal carbonyls due to backbonding.

• According to Molecular Orbital Theory (MOT), the lone pair on CO lies in a molecular orbital which has slight antibonding character.
• Since metals in higher oxidation have vacant d-orbitals, they tend to undergo backbonding when bonded with CO. This leads to the donation of the lone pair from an antibonding orbital, thereby increasing the bond order of CO. This in turn increases the IR stretching frequency.
• However, in the cases, when the metal does not vacant d-orbitals (for example, d¹⁰ systems) then the metal donates it's electrons to the LUMO (antibonding) of the CO, thereby reducing the bond order of CO. In such a situation, the IR stretching frequency of CO decreases to lower values.