The homoleptic cobalt carbonyls Co4(CO)12 and Co6(CO)16 are characterized by their equilibrium geometries, thermochemistry, and vibrational frequencies using density functional theory (DFT) methods with the B3LYP, BLYP, and BP86 functionals. The B3LYP predicted CoCo distances are 2.51 and 2.47 A for the C3v and Td structures, respectively, of Co4(CO)12. The global minimum for Co4(CO)12 has C3v symmetry with three bridging and nine terminal carbonyls. The 2.51 and 2.52 A CoCo distances suggest the single bond required for an 18-electron configuration for the metal atoms. This structure is close to an experimentally realized structure. A more symmetrical Co4(CO)12 structure with Td symmetry, analogous to that observed in the valence isoelectronic Ir4(CO)12 molecule, lies approximately 28 kcal/mol higher in energy and exhibits a small imaginary vibrational frequency ( approximately 40i). It has a slightly shorter CoCo distance of 2.47 A. Both Co4(CO)12 structures satisfy the 18-electron rule. The Co6(CO)16 structure has Td symmetry and satisfies the Wade-Mingos rules for an octahedral cluster. The nu(CO) carbonyl frequencies for both Co4(CO)12 and Co6(CO)16 computed with the BP86 functional are closer to the experimental values than those computed with the B3LYP and BLYP functionals. The structure of Co6(CO)16 is not known experimentally, but the BP86 functional predicts 2.56 A (CoCo), 1.77 and 2.02 A (CoC), and 1.66 and 1.20 A (CO) for the bond distances.