Homoleptic binary cobalt carbonyls with multiple cobalt-cobalt bonds have been examined theoretically using established levels of density functional methodology. These species include 19 structures ranging from the experimentally well characterized dibridged (CO)(3)Co(CO)(2)Co(CO)(3) to the proposed monobridged (CO)(2)Co(CO)-Co(CO)(2) structure with a formal quadruple bond. Consistent with experiment, three energetically low-lying equilibrium structures of Co-2(CO)(8) were found, of C-2 nu (dibridged), D-3d (unbridged), and D-2d (unbridged) symmetry. For Co-2(CO)(8), the BP86 method predicts the dibridged structure to lie 3.7 kcal/mol below the D-2d structure and 6.3 kcal/mol below the D-3d structure. The D-2d and D-3d structures thus have the opposite energetic ordering of that deduced from experiment by Sweany and Brown. A satisfactory harmony between theoretical and experimental vibrational frequencies and:IR intensities is found, although the D-2d and D-3d structures are essentially indistinguishable in this regard. For Co-2(CO)(7) the unbridged asymmetric structure suggested by Sweany and Brown is confirmed with the BP86 method, and with perhaps one exception the vibrational features agree well for theory and experiment For Co-2(CO)(6) only one vibrational feature has been assigned from experiment, but this band (2011 cm(-1)) fits very well with BP86 predictions for the monobridged D-2d symmetry structure with a formal Co=Co triple bond. For the Co-2(CO)(5) molecule, for which no experimental results exist. the most interesting structure is the monobridged closed-shell singlet with a very short (2.17 Angstrom) cobalt-cobalt bond, to which we assign a formal bond order of four. Potential energy distributions have been analyzed to identify the principal vibrations with cobalt-cobalt stretching contributions. The condensed phase Raman analysis by Onaka and Shriver of the Go-Co stretches for the three known isomers of Co-2(CO)(8) is remarkably consistent with the present predictions for the gas-phase species. Prospects for the synthesis of these and related dicobalt compounds are discussed.