It is, of course, extremely toxic, but that’s the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively.
Go read the whole thing. Yes, there are chemicals that scare even the less sane chemists. The series of posts is awesome.
UPDATE: Per infogalactic, a hypergolic propellant is one which spontaneously ignites when the two components come into contact with each other.