CO, NO and O2 as vibrational probes of heme protein interactions

► Negative correlations between FeX and XO stretching frequencies establish backbonding in XO adducts (X = C, N, O) of heme proteins. ► For Fe(II)CO adducts, donation from the trans axial ligand decreases νCO, with little change in νFeC, and shifts the backbonding correlation. ► For Fe(III)NO, Fe(II)NO and Fe(II)O2, dz2-dxz orbital mixing produces positive νFeX/νXO correlations, as the trans donation increases. ► For Fe(II)NO and Fe(II)O2, negative or positive correlations are induced by H-bonding to the outer or inner XO atom. ► Fe(III)NO adducts do not support distal H-bonds, but do support lone pair donor interactions. Strong donors interact with N, bending the FeNO.