The preparation and characterization of the beta-oxochlorin derivative [3,3,7,8,12,13,17,18-octaethyl-(3H)-porphin-2-onato(2 -)]copper(II), [Cu(oxoOEC)], and its,pi-cation radical derivative, [Cu(oxoOEC(.))][SbCl6], are described. Both compounds have been characterized by single-crystal X-ray structure determinations; IR, UV/vis/near-IR, and EPR spectroscopies; and temperature-dependent magnetic susceptibility measurements. Crystals of [Cu(oxoOEC)] have two crystallographically distinct molecules, one at a general position ([Cu(oxoOEC)]-gen) and a second at a special position ([Cu(oxoOEC)]-spe). [Cu(oxoOEC)]-gen has a S-4-saddled conformation whereas [Cu(oxoOEC)]-spe has a modest ruffled conformation. [Cu(oxoOEC(.))][SbCl6] shows a cofacial dimeric unit in the solid state, with a mean plane separation of 3.41 Angstrom and a lateral shift of 5.47 Angstrom. Crystal data for [Cu (oxoOEC)]: monoclinic, space group C2/c, Z = 12, a = 38.404(8) Angstrom, b = 14.692(6) Angstrom, c = 16.977(11) Angstrom, beta = 101.46(2)degrees Crystal data for [Cu(oxoOEC(.))][SbCl6]: triclinic, space group P (1) over bar, Z = 2, a = 13.063(1) Angstrom, b = 14.108(2) Angstrom, c = 11.486(2) Angstrom, alpha = 93.77(3)degrees, beta = 102.17(1)degrees, gamma = 74.07(1)degrees. The EPR spectrum of [Cu(oxoOEC(.))][SbCl6] in frozen dichloromethane at 77 K shows the characteristics of a dimeric copper(II) triplet state. Two broad, concentration-dependent near-IR "dimer bands" appear at 1285 and 1548 nm for [Cu(oxoOEC(.))][SbCl6]. Solid-state magnetic susceptibility measurements for [Cu(oxoOEC(.))][SbCl6] resulted in a large temperature dependence of the magnetic moments that can best be fit with a four-spin model. This model includes antiferromagnetic intermolecular copper-copper coupling (2J(Cu-Cu) = -70 cm(-1)), antiferromagnetic radical-radical coupling (2J(r-r) = -139 cm(-1)), and ferromagnetic intramolecular copper-radical coupling (2J(Cu-r) = 139 cm(-1)).