The ability of cobalt, rather than iron, to participate in the oxidative cleavage of porphyrins, a reaction related to that of the enzyme heme oxygenase, has been explored. Treatment of (octaethylporphyrin)cobalt(II) with dioxygen in the presence of ascorbic acid, as a reducing agent, in dichloromethane/tetrahydrofuran produces a green solution that can yield the cobalt verdoheme analogs [(OEOP)Co-II](PF?6 and {(OEOP)(CoCl2)-Cl-III} (OEOP is the monoanion of octaethyloxaporphyrin), {(OEB)Co}, a complex of the open chain tetrapyrrole octaethylbilindione (H(3)OEB), or octaethylbilindione itself. These products have been isolated and characterized by uv/vis and H-1 NMR spectroscopy. [(OEOP)Co-II](PF?6. CH2Cl2 (C36H45Cl2CoF6N6N4OP) crystallizes in the triclinic space group P (1) over bar, with a = 11.077(2) Angstrom, b = 14.033(2) Angstrom, c = 14.099(3) Angstrom, alpha = 104.56(2)degrees, beta = 101.70(2)degrees, and gamma = 110.70(1)degrees at 130 K with Z = 2. Refinement of 4171 reflections with F > 4.0 sigma(F) and 450 parameters yielded R = 0.066; R(w) = 0.060. The structure of the complex cation consists of a flat macrocycle with planar, four-coordinate cobalt at the center. These cations associate in pi-pi fashion with a mean-plane separation of 3.261 Angstrom and a short lateral displacement of 1.506 Angstrom. {(OEOP)(CoCl2)-Cl-III}. 2CH(2)Cl(2) (C37H47Cl6CoN4O) crystallizes in the monoclinic space group P2(1)/c, with a = 12.702(5) Angstrom, b = 8.125(3) Angstrom, c = 19.327(5) Angstrom, and beta = 101.23(2)degrees at 130 K with Z = 2. Refinement of 1803 reflections with F > 4.0 sigma(F) and 218 parameters yielded R = 0.057; R(w) = 0.054. The structure of the centrosymmetric complex consists of a six-coordinate cobalt ion with two axial chloride ligands and a planar oxaporphyrin which is disordered with regard to the location of the peripheral oxygen atom.