Phosphorus Trifluorodichloride, PF₃Cl₂

This compound is best prepared by mixing equal volumes of the trifluoride and chlorine confined over mercury, and allowing the mixture to stand for some days. The volume diminishes to one half, and the composition and molecular weight of the product are thus determined by Gay-Lussac's law and by the density, viz. 5.4 (air = l):—

PF₃ + Cl₂ = PF₃Cl₂

It is a colourless gas with an unpleasant odour, and does not burn or support combustion. It condenses to a liquid at -8° C. When heated to over 200° C. it decomposes according to the equation

5PF₃Cl₂ = 3PF₅ + 2PCl₅

It is much more easily reduced than the pentafluoride. Hydrogen combines with the chlorine at 250° C. leaving PF₃. Many metals, e.g. aluminium, magnesium, tin, lead and iron, decompose it similarly, forming chlorides but not decomposing the trifluoride. Sodium absorbs it completely. Sulphur combines with the phosphorus and the chlorine, giving PSF₃ and S₂Cl₂. Hydrolysis with a limited amount of water splits off the chlorine first, thus:—

PF₃Cl₂ + H₂O = POF₃ + 2HCl