UPSC Combined Geo-Scientist, Mains · 2021 · Mains Paper I

Q1(h). Carbonyl C-O bond strengths

Question

Arrange the following metal carbonyls in the increasing order of their C-O bond strength giving reason: [Cr(CO)₆], [Mn(CO)₆]⁺, [V(CO)₆]⁻.

Increasing C-O strength: [V(CO)₆]⁻ < Cr(CO)₆ < [Mn(CO)₆]⁺.

All three species are isoelectronic d⁶, 18-electron octahedral carbonyls: V(−I), Cr(0), Mn(I).
The electron-rich anion has the strongest metal-to-CO π back-donation; the cation has the weakest.
Back-donation populates CO π* orbitals and decreases C-O bond order, force constant and stretching frequency.
Thus ν(CO) follows the same increasing order as C-O strength; the trend in the back-bonding contribution to M-C strength is opposite.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Coversmetal carbonylsC-O bond strengthπ back-bondingCO stretching frequencysynergic bondingisoelectronic carbonyls