Cyanogen was a poisonous gas used by the Fourth Doctor to try and stop Solomon from resurrecting Morbius.
It could synthesized from hydrogen cyanide. (TV: The Brain of Morbius)
Behind the Scenes
This is an example of the show using real science. Cyanogen can, in fact, be produced from hydrogen cyanide via oxidation and a catalyst such as oxygen or hydrogen peroxide.[1]