Phosphonium bromide, dissociation

Phosphonium Bromide, PH4Br, was made by Serullas in 1831 from the component gases by direct union.14 Its dissociation pressure reaches only 1 atmosphere slightly below +38° C.13 It can also be made by passing phosphine into the most concentrated aqueous hydro-bromic acid,15 or hydrogen bromide dissolved in phosphoryl chloride,16 or from phosphine and bromine.14 It forms colourless cubic crystals which sublime at about +30° C.17 The heat of formation of the solid from the gases PH3 and HBr is +23 Cals., while that evolved when the initial materials are bromine (liquid), hydrogen and phosphorus (solid) is +44-1 Cals.15... 

The latter elass of compounds elicited a great interest because of their ability to entrap gas molecules, of which carbon dioxide is a remarkable example. Parieaud and co-workers reported an interesting insight on the dissociation conditions of CO2 from several ammonium salts and tetra-n-butyl phosphonium bromide ([P4444]Br) semiclathrate hydrates. Ye et al deseribed a comparison between phase equilibrium and morphology eharacteristics of hydrates formed by and [P444JCI... 

Infrared, Raman, x-ray and nentron diffraction studies have established the existence of tetrahedral PH cations in salts snch as PH4I. Phosphonium salts containing the tetrahedral PH4 cation (4.151) are generally less stable than the corresponding NHJ salts and dissociate more easily. The chloride and bromide easily form gases at room temperature and only the iodide, PH4I, mp= 18.5°C (vp = 50 mm at 20°C, and snblimes at 62°C), is crystalline, but it is decomposed by water to yield PH3. The PH cation is stable in solutions of strong acids such as H3O BFj and MeOH BF3, but the perchlorate is very explosive (above). 

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