A structural study of the high-spin five-coordinate complex iron(III) tetraphenylporphyrin azide Fe(TPP)N3 (3) and its six-coordinate low-spin adducts with 1-methylimidazole (1-Melm) and 1,2-dimethylimidazole (1,2-Me2Im) is presented. The latter two complexes were studied in order to explore the structural effect of steric strain induced by the presence of the 2-methyl substituent and to determine to what extent, if any, the set of complexes Fe-(TPP)(1-MeIm)N3 (4) and Fe(TPP)(1,2-Me2Im)N3 (5) mimic the structural differences observed in the vicinity of the metal in R-state HbO2 and T-state HbO2, respectively. Crystal data for 3 : triclinic space group P1BAR, a = 11.214(5) angstrom, b = 12.623(6) angstrom, c = 14.289(6) angstrom, a = 82.36(4)-degrees, beta = 68.78(3)-degrees, gamma = 66.72(3)-degrees, Z = 2,4119 unique reflections, R = 0.066. Crystal data for 4 : monoclinic space group P2(1)/n, a = 12.758(6) angstrom, b = 24.273(6) angstrom, c = 14,755(7) angstrom, beta = 106.42(3)-degrees, Z = 4, 3553 unique reflections, R = 0.086. Crystal data for 5 : monoclinic space group P2(1)/n, a = 13.045(4) angstrom, b = 24.059(29) angstrom, c = 14.694(5) angstrom, beta = 103.88(2)-degrees, Z = 4,3710 unique reflections, R = 0.094. The azide ligand in 3-5 is bonded in a bent fashion with a Fe-N-N angle of 122-degrees. The porphyrin core in 3 is domed with the iron displaced 0.46 angstrom from the plane of the four pyrrole nitrogens (P(N)). In 4 and 5 the porphyrin core displays approximate S4 ruffling with the metal located nearly in the PN plane. In going from 4 to 5 there is essentially no change in the Fe-N3 or Fe-N(p) (pyrrole nitrogen) bond distances. However, steric congestion is clearly manifested by (1) an increase of 0.083 angstrom in the Fe-N(Im) bond distance and (2) a tipping of the imidazole ligand in 5 by about 15-degrees from the symmetrical position observed in 4. These results and others are compared to the structural features of the analogous picket fence complexes Fe(TpivPP)(RIM)02(RIm = 1 -McIm and 2-MeIm) and are discussed with reference to possible structural changes that accompany the T-HbO2 to R-HbO2 switch.