The crystal and molecular structure of four-coordinate (octaethylporphinato)cobalt(II) has been determined at 127 K from X-ray diffraction data collected on an area detector. Crystals of Co(OEP) are isomorphous with Fe(OEP), one crystalline form of Ni(OEP), and Cu(OEP); the porphinato cores in the series are all planar. The M-N(p) bond distance order in the series is Fe > Co > Ni < Cu. The isomorphous M(OEP) derivatives form similar extended pi-pi aggregates with no metal atom dependence on the intermolecular geometry. A comparison of the analogous, isomorphous series of tetraphenylporphyrin compounds, which have significantly ruffled cores, is given. The average value of the Co-N(p) bond distance in Co(OEP) is 1.971 (6). Crystal data : C36H44N4Co, a = 13.046(15) angstrom, b = 13.217(12) angstrom, c = 4.742(7) angstrom, alpha = 90.60(6)degrees, beta = 92.56(2)degrees, gamma = 113.53(2)degrees, triclinic, space group P1BAR, V= 748.5 angstrom3, Z = 1, 2640 unique observed data, final data/variable = 14.1, R1 = 0.039, R2 = 0.044, all observations at 127 K.