Phosphorus halides

    Phosphorus forms the halides PX₃ (X = F, Cl, Br and I) and PX₅ (X= F, Cl and Br); PI₅ is unknown. Most are made by direct combination of the elements with the product determined by which element is in excess; PF₃, however, must be made by reaction below and a convenient synthesis of PF₅ is from KPF₆ (see below). The halides are all hydrolysed by water although PF₃ reacts only slowly.

    Each of the trihalides has a trigonal pyramidal structure above. Phosphorus trifluoride is a very poisonous, colourless and odourless gas. It has the ability (like CO) to form complexes with metals and Lewis acids such as BH₃, and its toxicity arises from complex formation with haemoglobin. Protonation of PF₃ can be achieved when HF/SbF₅ is used as the acid although an analogous reaction does not occur with AsF₃. [HPF₃][SbF₆]:HF is thermally unstable, but low-temperature structural data show that the tetrahedral [HPF₃]‏ ion has bond lengths of P^_H = 122 and P^_F =149 pm.

   The reaction of PF₃ with Me₄NF in MeCN gives [Me₄N][PF₄]. The [PF₄]^- ions are disphenoidal in shape, consistent with VSEPR theory, i.e. the structure is derived from a trigonal bipyramid with a lone pair of electrons occupying one equatorial position. In solution, [PF₄]^- is stereochemically non-rigid and the mechanism of F atom exchange is probably by Berry pseudo-rotation. When treated with an equimolar amount of water, [PF₄]^- hydrolyses according to equation below. With an excess of water, [HPF₅]^- also hydrolyses to [HPO₂F]^- making this the only product of the overall hydrolysis of [PF₄] ^-.

   Phosphorus trichloride is a colourless liquid (mp 179.5 K, bp 349 K) which fumes in moist air and is toxic.  Its reactions include those in scheme below POCl₃ is an important reagent for the preparation of phosphate esters.

    Single-crystal X-ray diffraction (at 109 K) data show that PF₅ has a trigonal bipyramidal structure, 14.32. In solution, the molecule is fluxional on the NMR spectroscopic timescale and a doublet is observed in the ¹⁹FNMRspectrum, i.e. all ¹⁹F environments are equivalent and couple with the ³¹P nucleus.   This stereochemical non-rigidity is another example of Berry pseudo-rotation. Electron diffraction data show that in the gas phase, PCl₅ has a molecular, trigonal bipyramidal structure (P^_Clax = 214, P^_Cl_eq = 202 pm), provided that thermal dissociation into PCl₃ and Cl₂ is prevented by the presence of an excess of Cl₂. In the solid state, however, tetrahedral [PCl₄]‏ (P^_Cl = 197 pm) and octahedral [PCl₆]^-(P^_Cl = 208 pm) ions are present, and the compound crystallizes with a CsCl lattice (Figure 5.16). In contrast, PBr₅ (which dissociates in the gas phase to PBr₃and Br₂) crystallizes in the form of [PBr₄]‏⁺Br^-. The mixed halide PF₃Cl₂ is of particular interest. It is obtained as a gas (bp 280 K) from the reaction of PF₃ and Cl₂ and has a molecular structure with equatorial Cl atoms. However, when PCl₅ reacts with AsF₃ in AsCl₃ solution, the solid product [PCl₄]‏[PF₆]^- (mp ≈ 403K) is isolated. Solid PI₅ has not been isolated,  but the isolation of the salts [PI₄]‏[AsF₆]^- (from the reaction of PI₃ and [I₃]‏[AsF₆^-) and [PI₄]‏[AlCl₄]^- (from the reaction between PI₃, ICl and AlCl₃) confirms the existence of the tetrahedral tetraiodophosphonium ion. The reaction of PBr₃ with [I₃][AsF₆] leads to a mixture of [PBr₄][AsF₆], [PBr₃I][AsF₆] and small amounts of [PBr₂I₂][AsF₆]. Selective formation of [PBr₄][AsF₆] can be achieved by treating PBr₃ with [Br₃]⁺‏[AsF₆]^-.

   Phosphorus pentafluoride is a strong Lewis acid and forms stable complexes with amines and ethers. The hexafluorophosphate ion, [PF₆]^- is made in aqueous solution by reacting H₃PO₄ with concentrated HF; [PF₆]^- is isoelectronic and isostructural with [SiF₆]^2-.  Salts such as [NH₄][PF₆] are commercially available, and [PF₆]^- is used to precipitate salts containing large organic or complex cations. Solid KPF₆ (prepared as in Figure 1.1) decomposes on heating to give PF₅ and this route is a useful means of preparing PF₅.

   Phosphorus pentachloride is an important reagent, and is made industrially by the reaction of PCl₃ and Cl₂; selected reactions are given in Figure 1.1.  Of the lower halides P₂X₄, the most important is the red, crystalline P₂I₄ (mp 398 K) which can be made by reacting white phosphorus with I₂ in CS₂. In the solid state, molecules of P₂I₄ adopt a trans (staggered) conformation .   In many of its reactions, P₂I₄ undergoes P^_P bond fission, e.g. dropping H₂O on P₂I₄ in an inert atmosphere produces [PH₄]I.

    Salts of [P₂I₅]‏⁺  can be obtained according to scheme 14.76. In these salts, the [P₂I₅]⁺‏ ion exists only in the solid state; the ³¹P NMR spectra of CS₂ solutions of dissolved samples show a singlet at≈178, consistent with the presence of PI₃ rather than [P₂I₅]‏⁺. In contrast, solution ³¹P NMR spectra have been obtained for [P₂I₅]‏⁺ in the presence of the [Al{OC(CF₃)₃}₄]^- anion.

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