Vibrational spectra of CO, NO, and O2 adducts of heme proteins contain information on interactions of the heme and its bound ligands with the surrounding protein matrix that may help in elucidating the mechanism of small-molecule activation. Whereas the heme−CO system is well studied and a framework exists for the interpretation of such interactions, heme−NO and −O2 complexes have not been systematically investigated. Here we examine resonance Raman spectra of all three classes of adducts, combining literature values with new data for FeIINO porphyrins having both electron-donating and electron-withdrawing substituents. Negative linear correlations are observed for all three adducts between their Fe−XO and X−O stretching frequencies. The slopes of these correlation lines are −0.4 for five-coordinate FeCO and FeNO porphyrins and −0.8 for five-coordinate FeO2 adducts. Thus, Fe−NO and Fe−O2 bonds are equally or even more sensitive than Fe−CO bonds to electronic influences that affect metal-to-ligand π back-b...