Phosphorus pentahalides

The phosphorus pentahalides can be made by the addition of halogen to the trihalides or to white phosphorus. These molecules may exist as covalent trigonal bipyramidal arrangements or in ionised tetrahedral form (3.41). The pentafluoride is the most stable it is dissociated a few percent at 100°C, and completely only at 300°C. The pentachloride, mp = 167°C, is considerably dissociated at 200°C and the pentabromide totally at 35°C. These compounds hydrolyse in at least two stages. 

Nuclear magnetic resonance studies have suggested the equivalence of all five bonds in PFj, but this has been explained on the basis of a rapid exchange of the non-equivalent F atoms (4.238). Such a process, which does not involve bond breaking, is known as Berry pseudorotation [14] (Chapters 3.2 and 13.3). It involves the simultaneous exchange of the axial F atoms with two of the equatorial F atoms, by merely changing the bond angles. 

Mass spectra of the pentachloride vapour indicate that it contains a small percentage of dimer molecules (4.239a) - species which may be present in some solvents at low temperatures. 

Solid phosphorus pentachloride, recrystaUised from nitrobenzene, has been shown by x-ray diffraction to consist of an assembly of tetrahedral PCIJ and octahedral PClg ions (4.239b). The same ions are present in methyl cyanide solution and in other solvents of high dielectric constant. Seven different ionic solid forms are now believed to exist. 

The greater P-Cl distance in PClg compared to that in PCI4 indicates the relative looseness of octahedral binding. This is supported by radioactive exchange experiments, and the preferential attack in fluorination reactions such as (4.250). 

This confirms the deductions from a gas phase electron-diffraction study (Z)3/, P-F 158 pm, P-F g 153 pm). However, the F nmr spectrum, as recorded down to — 100°C, shows only a single fiuorine resonance peak (split into a doublet by P- F coupling) implying that on this longer time scale (milliseconds, as distinct from instantaneous for electron diffraction) all 5 F atoms are equivalent. This can be explained if the axial and equatorial F atoms interchange their positions more rapidly than this, a process termed pseudorotation by R. S. Berry (1960) indeed, PF5 was the first compound to show this effect. The proposed mechanism is illustrated in Fig. 12.13 and is discussed more fully in ref. 91 the barrier to notation has been calculated as 16 2kJmol.  

The mixed chlorofluorides PCI4F (mp —59°, bp -1-67°) and PCI3F2 (mp —63°) are also trigonal bipyramidal with axial F atoms likewise PCI2F3 (mp —125°, bp -1-7.1°) has 2 axial and 1 equatorial F atoms and PCIF4 (mp —132°, 

LuckenbacH, Dynamic Stereochemistry of Pentaco-ordinate Phosphorus and Related Elements, G. Thieme, Stuttgart, 1973, 259 pp. 

PCI5 is even closer to the ionic-covalent borderline than is PF5, the ionic solid [PCUl+fPClg] melting (or subliming) to give a covalent molecular 

Minkwttz, J. Rohman, B. Steger, W. Wolfel and H. Oberhammer, Inorg. Chem. 25, 2828-35 

This confirms the deductions from a gas phase electron-diffraction study P-Fo 158 pm, 

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Phosphorus pentachloride, 79 40, 42-44 end use of chlorine, 6 135t sodium reactions with, 22 766 Phosphorus pentafluoride, 79 33 Phosphorus pentahalides, 79 31-33 Phosphorus pentoxide, 79 49, 69. See also Phosphorus(V) oxides as cellulose solvent, 77 272 in hydrogen fluoride manufacture, 74 11 vapor of, 79 49... 

Various structures have been found for phosphorus pentahalides ... 

The conversion of a 1,6-naphthyridinone into the corresponding halogeno-1, 6-naphthyridine is usually done by refluxing with phosphoryl chloride or bromide, with or without the addition of a tertiary base alternatively, a phosphorus pentahalide can be used but additional C-halogenation may result. Nontautomeric... 

The classical route to chloropyrazines is by treatment of a hydroxy-pyrazine with phosphoryl chloride bromopyrazines are similarly prepared by using phosphoryl bromide, phosphorus tribromide, or a mixture of both. Thus, treatment of hydro xypyrazine with phosphoryl chloride gives chloropyrazine in 92% yield,147 and treatment of the hydroxy compound with a mixture of phosphoryl bromide and phosphorus tribromide gives bromopyrazine in 58% yield.266 The use of phosphorus pentahalides frequently leads to substitution products (Scheme 22) for example, when hydroxypyrazine is treated with a mixture of phosphoryl bromide and phosphorus pentabromide both monobromo- and 2,6-dibromopyrazines are formed.287,268 Bromina-tion of hydroxypyrazine with bromine in the presence of small... 

Trimethylsilyl halides will exchange halogen with Me3N BX3 adducts (9) and BF4" (80). Phosphorus pentahalides (9, 13), A12C16 (9), and AsC13 (9) act similarly on amine adducts. 

Examples of molecules in which the central atom contains an expanded octet are the phosphorus pentahalides and sulfur hexafluoride. 

Phosphorus pentahalides PX5 (X = Cl and Br) are mild oxidants which were used already more than a century ago to oxidize platinum black to bridged dimers [Pt2(yU2-X)2X2(PX3)2] [235a]. Oxidative chlorination of Pt to PF complexes by PCI5 is classic, and compares favorably with use of CI2 which is common to carry out this reaction [235b]. 

Cholesteryl methyl ether is converted into 3)8-chloro- or 3) -bromo-cholest-5-ene by the appropriate phosphorus pentahalide in the presence of boron trifluoride etherate. ... 

For the above reasons nitrogen forms many compounds of types not formed by other elements of this group, and for this reason we deal separately with the stereochemistry of this element. For example, the only compounds of N and P which are structurally similar are the molecules in which the elements are 3-covalent and the phosphonium and ammonium ions. There are no nitrogen analogues of the phosphorus pentahalides, and there is little resemblance between the oxygen compounds of the two elements. Monatomic ions of nitrogen and phosphorus are known only in the solid state, in the salt-like nitrides and phosphides of the more electropositive elements. The multiple-bonded azide ion, N3, is peculiar to nitrogen. 

Reaction with alcohols and phenols. Horner and co-workers introduced use of the reagents for the preparation of alkyl and aryl halides from alcohols or phenols, and Wiley and co-workers established that the reagents have considerable advantage over phosphorus pentahalides in the C—OH—C—X conversion and discussed the mechanism. For the preparation of n-butyl bromide the Wiley group added bromine under nitrogen to a solution of n-butanol and triphenylphosphine in dimethyl-formamide until 2 drops persisted in giving the solution an orange tint. Volatile... 

The halides and oxyhalides of some nonmetals hydrolyze (react with water) to produce two acids a (binary) hydrohalic acid and a (ternary) oxyacid of the nonmetal. Phosphorus trihalides react with water to produce the corresponding hydrohalic acids and phosphorous acid, a weak diprotic acid, whereas phosphorus pentahalides give phosphoric acid and the corresponding hydrohalic acid. 

The classical synthesis of imidoyl chlorides involves the reaction of N-substituted carboxylic acid amides with phosphorus pentahalides. Wallach and his students (288-292 1876-1882 investigated this reaction... 

If an excess of phosphorus pentahalides is used on aliphatic carboxylic acid amides, having a-hydrogens, further halogenation occurs. For example, the reaction of propionic acid ethylamide (XV) with 3 equivalents of phosphorus pentachloride yields the imidoyl chloride XVI, which can also be obtained from a,a-dichloropropionic acid ethylamide (XVII) and one equivalent of phosphorus pentachloride (5 ). 

Similarly, reaction of cyclic imides with phosphorus pentahalides should afford the corresponding dihaloimidoyl halides. In 1882 Bernheimer ( ) treated glutarimide with phosphorus pentachloride, and he assigned the imidoyl chloride structure XLIV to the reaction product. However, it was later shown that the compound was 2 3,6-trichloropyridine (XLV) ( ). 

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