Reaction of [Co(C₅R¹₅)(CO)₂](R¹= H or Me) with 1,2,3-selenadiazoles in toluene under reflux in the presence of an excess of elemental selenium leads in moderate yield to the diselenolenes [Co(C₅R¹₅){SeC(R²)C(R³)Se}][R²–R³=(CH₂)₆, CHCH(CH₂)₄ or CHCH(CH₂)₂CHCH; R²= Ph, R³= H; R²= R³= Ph]. In particular this procedure provides access to novel compounds without electron-withdrawing substituents. The products have been characterised by ¹H NMR, IR, UV/VIS and mass spectroscopy, microanalysis and cyclic voltammetry. Each diselenolene undergoes a fully reversible one-electron reduction, and exhibits a low-energy electronic transition at ca. 800 nm. Of the substituents (R¹,R²,R³), R¹ has the greatest influence on the spectroscopic and electrochemical properties. For structurally similar compounds there is a linear correlation between the half-wave potential for the reduction and the energy of the first electronic absorption band.