Three isomeric forms of dicobalt octacarbonyl, [Co-2(CO)(8)], with C-2v, D-3d and D-2d point group were optimized by using density functional theory (DFT/B3LYP) method with LANL2DZ basis set for the cobalt atoms and 6-31G(d) basis set for the other atoms in the gas phase. Electronic structures, carbonyl stretching frequencies and Mulliken population analysis were determined from the optimized structures. Electronic structures indicate that each of the dicobalt octacarbonyl isomers have been constituted from two trigonal bipyramidal geometry. While the isomer C-2v has two bridged carbonyl groups, in the isomers D-3d and D-2d all carbonyl groups are coordinating as terminal. The calculated C-O stretching frequencies are in a good agreement with experimental frequencies. Experimental C-O stretching frequencies were assigned to isomers according to the calculated frequencies. Mulliken population analysis show that free carbonyl ligands transfer their electron to the cobalt atoms during formation of the complexes. The electronic spectra of isomers were obtained by using time dependent density functional theory (TD-DFT/B3LYP) method with LANL2DZ basis set for the cobalt atoms and 6-31G(d) basis set for the other atoms in the gas phase. The theoretical electronic spectra of isomers are in a good agreement with experimental spectra. The calculated bands at 277.9, 278.1 and 284.1 nm for isomers C-2v, D-2d and D-3d were assigned to metal-ligand charge transfer transitions and the shoulder at 344.6 nm was assigned to pure metal center transitions for isomer D-3d. (C) 2014 Elsevier B.V. All rights reserved.