Abstract; When crystals of (2-methylimidazole)-mcro-tetra (a, a, a, ao-pivalamidophenyl) porphyrinatoiron (ll)-ethanol, Fe (TpivPP)(2-MeIm)-EtOH, are exposed to dioxygen, the crystals of the resultant dioxygen adduct are still suitable for diffraction studies. The direct, precise determination of the stereochemical changes accompanying oxygenation of an iron (ll)-(porphyrinato)(base) complex has been carried out usingconventional X-ray diffraction methods. The structures have been refined by full-matrix, least-squares methods, using 4176 and 5183 reflections for the deoxy and oxy complexes, respectively, to R indices on F2 of 0.162 and 0.120. For the portionof data where Fa2> 3ff (F02) the respective indices on F are 0.086 and 0.083. Crystal data for the deoxy compound follow: space group C\h-C2/c, Z= 4, molecular symmetry C2, a= 18.871 (11) Á, b= 19.425 (13) Á, c= 18.434 (11) Á, ß= 91.48 (3), V= 6755.0 Á3. The oxy complex is nearly isomorphic with Z= 4 in space group C2/c with a cell of dimensions a= 18.864 (5) Á, b= 19.451 (5) Á, c= 18.287 (5) Á, ß= 91.45 (2), V= 6707.0 Á3. Some selected parameters for the coordination spheres, with those in square brackets pertaining to the dioxygen adduct, follow: Fe-Np0rPh= 2.068 (5), 2.075 (5)[1.997 (4), 1.995 (4)] A; Fe-Nlm= 2.095 (6)[2.107 (4) 1 A. The iron atom is displaced 0.399 [0.086] A from the least-squares plane of the porphinato nitrogen atoms toward the imidazole ligand. The Fe-O separation is 1.898 (7) Á. The average 0-0 separation is 1.22 (2) A and the Fe-0-0 angle is 129 (1), In the presence of ethanol the deoxy complex binds dioxygen reversibly, noncooperatively, and with lower affinity thanwhen the sample is de-solvated—in the latter case dioxygen uptake has been found to be cooperative. The structure and properties of these possible models for T-state deoxy-and oxyhemoglobin are correlated and then compared with the 1-methylimidazole analogue. The sterically active 2-methyl substituent appears to perturb the Fe-O bond but not the Fe-N| m bond.