Reaction of dicobalt octacarbonyl with elemental red selenium at room temperature in THF solution gives rise to Co-3(CO)(9)(mu(3)-Se) (1), Co-4(CO)(10)mu(4)-(zSe)(2) (2), [(mu(3)-Se)Co-3(CO)(7)](2)(mu(4)-Se-2) (4), and CO6(mu(3)-Se)(8)(CO)(6) (5) The complexes 4 and 5 have been characterized by a single-crystal X-ray diffraction analysis. Crystal data for 4 are as follows : triclinic space group P $($) over bar$$ 1, a = 9.649(2) Angstrom, b = 12.688(3) Angstrom, c = 12.952(6) Angstrom, alpha = 66.61(3)degrees, beta = 71.89(3)degrees, gamma = 83.53(2)degrees, V = 1383.1(8) Angstrom(3), Z = 2, rho(calcd) = 2.549 Mg/m(3). The structure was solved by direct methods and refined (1646 reflections, F > 4.0 sigma(F)) to the final residuals R = 0.084 and wR = 0.107. The molecule consists of two (mu(3)-Se)Co-3 pyramids, joined by a Se-2 group which bridges one Go-Co edge of each cluster. The Co2Se2Co2 array is folded along the diselenide bond (119 degrees), and the two mu(3)-Se ligands are on the opposite side with respect to the Se-Se hinge. Complex 5 crystallizes in the rhombohedral R $($) over bar$$ 3 space group, with a = 24.915(4) Angstrom, c = 12.655(3) Angstrom, V = 6803(2) Angstrom(3), Z = 9, and rho(calcd) = 2.877 Mg/m(3). X-ray data were collected using a crystal sealed with mother liquor, and the structure was solved by direct methods and refined (1449 reflections, F > 4 sigma(F)) to the final R = 0.045 and wR = 0.053. The molecule can be described as a Co-6 octahedron, with a centrosymmetric crystallographic symmetry, whose faces are bridged by Se atoms, forming an Ses cubic array. One CO group is terminally bonded to each metal atom. A new complex has been isolated from the products of the reaction between Co-2(CO)(8) and CSe2 in petroleum ether. It has been identified by infrared spectroscopy as Co-6(mu(4)-Se)(mu(6)-C-2)(CO)(14) (6), where the six cobalt atoms are arranged in a "boat" configuration and include a semi-interstitial C-2 unit. A mu(4)-Se atom and 8 terminal and 6 bridging CO groups complete the structure. Infrared arid Raman spectra of the complexes are discussed, and the main vibrational modes are assigned and compared with those of the analogous sulfur complexes.