A (phenylenediselenolato)cobalt complex dimer, [Co(eta(5)-C5H5)(Se2C6H4)](2) (1), was synthesized by a reaction of carbonyl(eta(5)-cyclopentadienyl)diiodocobalt(III) ([Co(eta(5)-C5H5)I-2(CO)]) with poly(o-diselenobenzene). The structure of 1, determined by single-crystal X-ray crystallography, was found to be located in the space group of P2(1)/c (No. 14), with a = 9.3346(5) Angstrom, b = 11.6477(g) Angstrom, c = 10.2179(5) Angstrom, beta = 111.491(1)degrees, and Z= 2. Covalent Co-Se bonds bridge the metal centers. In solution, dimers and monomers coexist at equilibrium. The dissociation equilibrium constant of 1 in solution was evaluated by H-1 NMR spectra at several temperatures between 20 and 80degreesC. Dissociation enthalpies/entropies were found to be 50/110, 60/120, and 88 W mol(-1)/200 J K-1 mol(-1) in dimethyl sulfoxide-d(6), benzene-d(6), and chloroform-d(1), respectively.