The binuclear cyclopentadienylcobalt carbonyls Cp2Co2(CO)(n) (n = 3, 2, 1; Cp = eta(5)-C5H5) are studied by density functional theory using the B3LYP and BP86 functionals. The experimentally known monobridged isomer Cp2Co2(CO)2(U-CO) and the tribridged isomer Cp2Co2(mu-CO)(3) of Cp2Co2(CO)(3) with formal Co-Co single bonds are found to be similar in energy, with the precise relative energies of the two isomers depending on the functional chosen. For Cp2Co2(CO)2, the experimentally known coaxial isomer Cp2Co2(mu-CO)(2) with two bridging CO groups and a formal Co=Co double bond (2.360 angstrom by B3LYP or 2.346 angstrom by BP86) is found to lie 38.2 (B3LYP) or 34.9 kcal/mol (BP86) below a perpendicular isomer perpendicular to-Cp2Co2(CO)(2). Similarly, for Cp2Co2(CO), the coaxial isomer Cp2Co2(mu-CO) with one bridging CO group and a formal Co&3bond; Co triple bond (2.021 angstrom by B3LYP or 2.050 angstrom by BP86) is found to lie 9.36 (B3LYP) or 9.62 kcal/mol (BP86) below the corresponding perpendicular isomer perpendicular to-Cp2Co2(CO). This coaxial isomer Cp2Co2(mu-CO) is a possible intermediate in the known pyrolysis of the trimer (eta(5)-C5H5)(3)Co-3(mu-CO)(3) to give the tetranuclear complex (eta(5) -C5H5)(4)Co-4(mu(3)-CO)(2). These optimized Cp2Co2(CO)(n) (n = 3, 2, 1) structures can be compared with the corresponding Fe-2(CO)(6+n) structures since the CpCo and Fe(CO)3 groups are isolobal. In general, the metal-metal bonds are 0.09-0.22 A shorter for the Cp2Co2(CO), (n = 3, 2, 1) complexes than for the corresponding Fe-2(CO)(6+n) complexes. For Fe2(CO)9, the experimentally well-known Fe-2(CO)(6)(mu-CO)(3) isomer is shown to be very close in energy to the unknown Fe-2(CO)(8)(mu-CO) isomer, with the precise relative energies depending on the basis set used.