We have synthesized and studied via solid-state NMR, Mössbauer spectroscopy, single-crystal X-raydiffraction, and density functional theory the following Fe-O
2 analogue metalloporphyrins: Fe(5,10,15,20-tetraphenylporphyrinate) (nitrosobenzene)(1-methylimidazole); Fe(5,10,15,20-tetraphenylporphyrinate) (nitrosobenzene)(pyridine); Fe(5,10,15,20-tetraphenylporphyrinate)(4-nitroso-
N,
N-dimethylaniline)(pyridine); Fe(2,3,7,8,12,13,17,18-octaethylporphyrinate) (nitrosobenzene)(1-methylimidazole) and Co(2,3,7,8,12,13,17,18-octaethylporphyrinate)(NO). Our results show that the porphyrin rings of the two tetraphenylporphyrinscontaining pyridine are ruffled while the other three compounds are planar: reasons for this are discussed.The solid-state NMR and Mössbauer spectroscopic results are well reproduced by the DFT calculations, whichthen enable the testing of various models of Fe-O
2 bonding in metalloporphyrins and metalloproteins. Wefind no evidence for two binding sites in oxypic
ket fence porphyrin, characterized by very different electricfield gradients. However, the experimental Mössbauer quadrupole splittings can be readily accounted for byfast axial rotation of the Fe-O
2 unit. Unli
ke oxymyoglobin, the Mössbauer quadrupole splitting inPhNO
myoglobin does not change with temperature, due to the static nature of the Fe
PhNO subunit, as verifiedby
2H NMR of Mb
[
2H
5]PhNO. Rotation of O
2 to a second (minority) site in oxymyoglobin can reduce theexperimental quadrupole splittings, either by simple exchange averaging, or by an electronic mechanism, withoutsignificant changes in the Fe-O-O bond geometry, or a change in sign of the quadrupole splitting. DFTcalculations of the molecular electrostatic potentials in CO, PhNO, and O
2-metalloporphyrin complexes showthat the oxygen sites in the PhNO and O
2 complexes are more electronegative than that in the CO system,which strongly supports the idea that hydrogen bonding to O
2 will be a major contributor to O
2/CO discriminationin heme proteins.