The X-ray characterization of the five-coordinate picket-fence porphyrin complex, [Co(TpivPP)(2-MeHIm)], is reported. The complex has the displacement of cobalt from the porphyrin plane = 0.15 A, and Co-N(Im) = 2.145(3) and (Co-N(p))(av) = 1.979(3) A. This five-coordinate complex, in the presence of dioxygen and excess 2-methylimidazole, undergoes an unanticipated, photoinitiated atropisomerization of the porphyrin ligand, oxidation of cobalt(II), and the formation of the neutral cobalt(III) complex [Co(alpha,alpha,beta,beta-TpivPP)(2-MeHIm)(2-MeIm(-)]. Two distinct examples of this complex have been structurally characterized, and both have structural parameters consistent with cobalt(III). The two new Co(III) porphyrin complexes have axial Co-N(Im) distances ranging from 1.952 to 1.972 A, but which allow for the distinction between imidazole and imidazolate. An interesting intermolecular hydrogen bonding network is observed that leads to infinite helical chains. UV-vis spectroscopic study suggests that [Co(TpivPP)(2-MeHIm)(O(2))] is an intermediate state for the oxidation reaction and that the atropisomerization process is photocatalyzed. A reaction route is proposed based on the spectroscopic studies.