Cyanogen

Cyanogen was a poisonous gas used by the Fourth Doctor to try and stop Solon from resurrecting Morbius.

It could be synthesized from hydrogen cyanide, also known as prussic acid. (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.