The O2 vibration of dioxygen adducts of Fe and Co model complexes of alpha,alpha,alpha,alpha-tetrapivalamidophenylporphyrin ("picket fence" porphyrin, TpivPP) with 1-methylimidazole and 1-tritylimidazole as axial bases are reported, obtained with difference techniques between 16O2, 18O2, 169-18O, and NO with a Fourier transform infrared spectrometer. Assignments of upsilono2 are (O2)Fe(TpivPP) 1-methylimidazole, 1159 cm-1 in Nujol; (O2)Fe(TpivPP) 1-tritylimidazole, 1163 in benzene; (O2)Co(TpivPP) 1-methylimidazole, 1150 in Nujol; (O2)Co(TpivPP) 1-tritylimidazole, 1153 in benzene. Comparisons with other known Fe, Co, Cr, and Ti dioxygen complexes are made, and it is concluded that the bent dioxygen ligand is best viewed as bound superoxide, O2-. The CO affinities of various hemoproteins and model systems are discussed. A correlation between the CO stretching frequency and its binding constant is described. The drastically lowered affinity of hemoproteins for CO compared with unencumbered models is attributed to steric hindrance in the distal binding site, which allows discrimination between the already bent FeIII-O2- and the normally linear FeII-CO systems. If the affinity of hemoproteins in living systems for CO relative to O2 were not decreased, then massive poisoning would result from endogenous CO.