Cyanogen

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

It could be synthesised 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 (also called "prussic acid" in the real world) via oxidation and an oxidising agent such as oxygen or hydrogen peroxide.