The structure of a solvate of mono (l-methylimidazole)(dioxygen)-meiotetrakis (a, a, a, ao-pivalamidophenyl) porphinatoiron (II),[Fe (TpivPP)(l-Me-imid)(02)]-!/2 (C6H6)'1/2 (N2C4H6), has been determined using three-dimensional x-ray diffraction data collected by counter methods. Crystal symmetry is consistent with the monoclinic space groups C2/c or Cc. The unit cell parameters are a= 18.690 (3) Á, b= 19.154 (3) Á, c= 18.638 (3) Á, ß= 91.00 (1), and Z= 4. Least-squares refinement of the structure in space group C2/c has led to a conventional R factor (on F) of 0.109 using 1784 reflections with I> a¡. The crystal structure consists of monomeric units of Fe (TpivPP)(l-Me-imid)(02) with benzene or 1-methylimidazole molecules occupying one solvate site in the asymmetric unit. Dioxygen is coordinated to the iron atom in the end-on bent bond mode. The Fe-0 separation is 1.75 (2) Á. As a consequence of the twofold symmetry imposed on the molqcule, the 1-methylimidazole group and the terminal oxygen atom are disordered. Additional disorder is found for the tert-butyl groups and for the terminal oxygen atom which can also occupy either of two crystallographically independent sites andwhich thus exhibits overall fourfold disorder. The two crystallographically independent 0-0 separations are 1.15 (4) and 1.17 (4) Á and the respective Fe-0-0 bond angles are 133 (2) and 129 (2). The 0-0 separations may be underestimated by as much as 0.15 A. The Fe-N,^ separations are 1.99 (1) and1. 97 (1) A and the iron atom is displaced 0.03 A from the mean plane of the porphinato nitrogen atoms toward the dioxygen ligand. The Fe-Nimy separation is 2.07 (2) A. Precise structure analysis is prevented by limited data of poor quality and by the disorder and high thermal motion of many parts of the structure.