The sterically encumbered OC(3)OPor system consists of a 1,2,4,5-substituted benzene cap with five-atom arms of the type -O(CH2)(3)O- linking the benzene cap to the ortho positions of 5,10,15,20-tetraphenylporphyrin. The structures of the following compounds have been determined by single-crystal X-ray diffraction methods : H-2(OC3-OPor) (1), Fe(OC(3)OPor)(Cl) (2), Fe(OC(3)OPor)(CO)(1-MeIm) (3), and Fe(OC(3)OPor)(CO)(1,2-Me(2)Im) (4). Structures 1-3 pack as one crystallographically independent porphyrin with solvate molecules, whereas structure 4 packs with half of a crystallographically independent porphyrin molecule and a solvate molecule. In compound 2 the Cl ligand is bound to the Fe center on the unprotected side of the porphyrin. In 3 and 4, which represent R-state (relaxed) and T-state (tense) models for hemoglobin, respectively, CO is bound underneath the cap and either 1-MeIm (3) or 1,2-Me(2)Im (4) is bound to the sixth coordination site, opposite the CO and outside the cap. The bulkier 1,2-Me(2)Im in 4 forces the Fe atom 0.10 Angstrom out of the mean nitrogen plane toward the 1,2-Me(2)Im ligand. In 3, where the base is less bulky, the Fe atom lies 0.06 Angstrom out of the plane toward the CO ligand. The cap-to-porphyrin distance increases approximately 0.8 Angstrom to accommodate CO, from 4.74 and 4.65 Angstrom in 1 and 2, respectively, to 5.55 and 5.59 Angstrom in 3 and 4, respectively. The Fe-C-O angle is 173.9(7)degrees in 3 and is constrained by symmetry to be 180 degrees in 4.