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Phosphorus(III)

All four symmetrical trihalides are well known: gaseous PF, the yellowish liquids PCl and PBr, and the solid PI. These materials are moisture sensitive, hydrolysing to give phosphorous acid. The trichloride, a common reagent, is produced by chlorination of white phosphorus:

The trifluoride is produced from the trichloride by halide exchange. PF is toxic because it binds to haemoglobin.

Phosphorus(III) oxide, P O (also called tetraphosphorus hexoxide) is the anhydride of P(OH), the minor tautomer of phosphorous acid. The structure of P O is like that of P O without the terminal oxide groups.