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Model iron(II) porphyrin complexes for the active site of myoglobin and oxymyoglobin have been synthesized and fully characterized by Mossbauer, electronic, and infrared spectral analysis, magnetic susceptibility, and X-ray crystallography. The synthesis is reported for the "picket fence porphyrin," mero-tetra(a,a,a',a-o-pivalamidophenyl)porphyrin, which has great steric bulk creating a nonprotic cavity on one side of the porphyrin. The unsaturated ferrous complex, 1, reacts with strong field ligands to give six-coordinate, diamagnetic complexes, 2. The complexes, 2, react reversibly with oxygen, in solution at 25°. Crystalline diamagnetic dioxygen complexes, 3, having 1-methylimidazole or 1-n-butylimidazole as axial ligands were isolated and characterized. The complex, 3, contains O2 bound "end-on" with an Fe-O-O angle of 136 (4)°. A dioxygen complex with tetrahydrofuran as an axial base, 4, was also prepared and appears to be paramagnetic (2.4 BM). Carbonyl complexes with imidazoles (i-co = 1965 cm-1) and THF (veo = 1955 cm-1) as axial ligands are all diamagnetic.The dioxygen complexes, 3 and 4, show no vo2 at 25°but a strong, sharp band is seen at 1385 cm-1 at -175°. The complexes are thus described as containing coordinated singlet ( ) oxygen. The remarkable stability of the complexes, 3 and 4, and their Mossbauer and spectral properties are discussed.

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be transmitted through this system and would not affect the coupling of 13C-2 to H-5". The large amount of electron density in the C-2-0-2 bond of 2 is extended and the Medical Research Council of Canada (Grant MA-3406 to . N. G. J.). We wish to thank Dr. Brian D. Sykes for valuable discussions regarding the anomalous couplings for compound 2.