A second, trigonal, phase of [CCo3(CO)9]2, mu6-dicarbide-1:2:3kappa3C1;4:5:6kappa3C2-hexakis(tricarbonylcobalt)-(6 Co-Co), has been found, in addition to the monoclinic one reported earlier [Brice & Penfold (1972). Inorg. Chem. 11, 1381-1384]. The intermolecular geometry of the molecule in the two crystallographic modifications is virtually the same. The two Co3 triangles are twisted by ca 28-degrees with respect to each other, indicating a conformation between staggered and eclipsed. Optically active crystals of the trigonal phase form by spontaneous resolution of the enantiomorphic conformers.