The molecular structures of the complexes of Co(OEP)CH3 Co(OEP)(CH3)(Py), and Co(OEP)(DMAP) were determined by X-ray diffraction. The complexes are related to each other and to coenzyme B-12 by a reaction sequence that involves coordination of pyridine to the five-coordinate alkyl-Co(III) porphyrin complex to afford a six-coordinate alkyl-Co(III) porphyrin pyridine complex followed by homolysis of the alkyl-cobalt bond to afford a five-coordinate Co(II) porphyrin pyridine complex. The compound (2,3,7,8,12,13,17,18-octaethylporphyrinato)(4-(N,N’-dimethylamino)pyridine)cobalt(II), Co(OEP)(DMAP); crystallized in the monoclinic space group P2(1)/n (Z = 4) with unit cell dimensions a = 14.392(6) Angstrom, b = 14.864(6) Angstrom, c = 18.217(5) Angstrom, beta = 95.60(3)degrees, and V = 3878.4(2.6) Angstrom(3). The structure was refined to R = 0.050 on F-o(2) on the basis of 5122 reflections with I>0. The 1.982(2)-Angstrom Co-N-eq distance and the 0.12-Angstrom displacement of the cobalt atom from the porphyrin nitrogen plane are typical of five-coordinate, square-pyramidal complexes of cobalt(II) porphyrins with nitrogenous bases, but the 2.191(2)-Angstrom Co-N-ax distance is longer than previously observed for a pyridine axial ligand. The compound (2,3,7,8,12,13,17,18-octaethylporphyrinato) (methyl)cobalt(III), Co(OEP)CH3, crystallized with two independent molecules per asymmetric unit in the triclinic space group (Z = 4) and had unit cell dimensions a = 13.776(1) Angstrom, b = 14.038(2) Angstrom, c = 16.593(2) Angstrom, alpha = 99.20(1)degrees, beta = 90.07(1)degrees, gamma = 95.67(1)degrees, and V = 3151.6(6) Angstrom(3). The structure refined to R = 0.061 on F-o on the basis of 4699 reflections with I > 2 sigma(I). The agreement of structural details between the two independent molecules was excellent, despite some disordering of ethyl groups. The cobalt atom has square pyramidal geometry and mean displacement of 0.10 Angstrom from the plane of the four nitrogen atoms. The average Co-C and Co-N-eq distances are 1.973 and 1.966 Angstrom, respectively. The compound (2,3,7,8,12,13,17,18-octaethylporphyrinato)(methyl)(pyridine)cobalt(III), Co(OEP)-(CH3)(Py), crystallized in the triclinic space group (Z = 2) with unit cell dimensions a = 10.667(2) Angstrom, b = 10.902(2) Angstrom, c = 16.119(3) Angstrom, alpha = 97.51(3)degrees, beta = 97.67(3)degrees, gamma = 99.96(3)degrees, and V = 1808.5(6) Angstrom(3).