Cobalt diselenide was a biogenic weapon that affected the nervous system; while deadly to Cardassians, it was harmless to most other humanoids. The agent could be synthesized from selenium and rhodium nitrite, and was required to be kept at a low temperature.

In 2373, Michael Eddington of the Maquis synthesized a large quantity of cobalt diselenide and introduced it to the Cardassian colonies on Veloz Prime and Quatal Prime. The Breen on Portas V provided the refrigeration units necessary to keep the weapons stable. Eddington eventually turned over his remaining cobalt diselenide to Starfleet after Captain Benjamin Sisko threatened to poison all the Maquis colonies in the Demilitarized Zone with trilithium resin in retaliation. (DS9: "For the Uniform")

Only selenium and rhodium nitrate were mentioned in the concoction of this substance.
In the real world, CoSe2 is a non-volatile crystalline solid – its toxicity is not currently known. Zhang et al. describe its synthesis and characterization in Materials Research Bulletin, 2000, 35, 2403-8. [1]
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