Hexafluorine was a gas, deadly to Humans.
In 2153, Enterprise found the Vulcan cruiser Seleya in an asteroid field. When on board, Archer, T'Pol, Reed, and Hawkins were attacked by zombie-like Vulcans affected by trellium-D. After finding the Seleya's engine room, they attempted to contact Enterprise. Although the Starfleet ship managed to get through to them, the communication channel was cut short by the Vulcans, who then flooded the engine room with hexafluorine gas, forcing the team to escape. (ENT: "Impulse")