Can the FeCO bending be higher than the FeC stretching frequency in CO adducts of heme proteins?

We have performed density functional quantum chemical calculations on a CO-bound iron-porphyrin complex and derived a plausible mechanism to explain why the FeCO deformation frequency is higher in the saturated heme–CO systems than in the unsaturated triatomic FeCO molecule. The suggested MO-based mechanism also provides physical grounds for the experimentally observed δ(FeCO) ≥ ν(FeC) trend as well as for the large deformability of the FeCO unit in heme proteins.