Mixed Pentahalides

Like the other pentahalides, the mixed pentahalides can either be based on a covalent trigonal bipyramid or exist in ionised forms. Isomeric arrangements of the substituents on the trigonal 

The NMR spectra of the mixed halides indicate that they contain F atoms in only one kind of environment. A pseudorotation process is therefore necessary to secure equivalence between axial and equatorial F atoms in the case of PF3CI2 and PF4CI (4.264). At low temperatures however, these pseudorotations disappear, and the NMR spectra indicate non-equivalent F atoms in PF4CI below -138°C and in PF3CI2 below -80°C. 

Mixed fluorohalides can be obtained by addition of halogen to PF3 (4.265). They are thermally unstable colourless gases which tend to dissociate, the bromine componnd being the least stable (4.266). Phosphorus tetrafluorochloride can be made in about 52% yield by a reaction between PFj and BCI3 at low pressure (4.267). 

3PCI4+ PFg +TKOH KPF6+K2HPO4+4KCI+3H2O 

Hydrotetrafluorophosphorane, HPF4, mp = -100°C, and dihydrotrifluoro phosphorane, H2PF3, mp = -52°C, can be made by reacting hydrogen fluoride under anhydrous conditions with phosphorous and hypophosphorous acids, respectively (4.271 through 4.272). Alternatively, these phospho-ranes may be obtained from PF, by reactions (4.273) and (4.274) [26,27]. 

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Mixed pentahalides are more readily isolated and are of at least three types ionic, tetrameric, and less stable molecular trigonal-bipyramidal monomers. Thus, chlorination of a mixture of ASF3/ASCI3 with CI2, or fluor-ination of AsCU with CIF3 (p. 828) gives [AsCl4]" [AsF6] [mp 130°(d)] whose X-ray... 

Phosphoryl chloride (b.p. 105 °C) has been used extensively as a nonaqueous solvent (see Chapter 10). Reactions of PC13 with other halogens give mixed pentahalides. 

Oxidizing agents such as H2O2 and OCF can be employed in this reaction. Reactions of PC13 with other halogens yield pentahalides that contain two different halogens. For example, a mixed pentahalide can be prepared by the reaction... 

It combines vigorously with all the halogens giving mixed pentahalides (q.v.). It is hydrolysed very slowly by cold water, rather more quickly by boiling water, and quickly by aqueous alkalies, giving in the last case alkali fluoride and phosphite. It was considered by Moissan 3 and by Berthelot 4 that the first product of hydrolysis was a fluophos-phoric acid (q.v.), the potassium salt of which was fairly stable. 

The other pentahalides all have complex structural behavior centering around the existence of fairly stable EX4 ions. Not all of the possible binary pentahalides are known, but there are quite a few mixed pentahalides. All of the binary ones and the better known mixed ones are listed in Table 10-4. Some of the important structural features will now be discussed. 

Mixed pentahalides, PF5 X (X = Cl, Br, 1), are known but only the chlorides PF4CI, PF3CI2, PF2CI3, and PF3Br2 are well characterized as molecular species. Many of the other mixed halides are complex salts or mixtures of complex salts of the type PX4+X- and PX4+ PXg. ... 

The mixed arsenic trihalides AsX X3 and pentahalides AsX X 3 are essentially unstable compounds. But it has been observed that mixtures of AsCls and AsR favor the formation of AsClsI according to equation (16). A mixture of AsFs and AICI3 has been shown to form small amounts of AsFs Cl and AsFCls by NMR. In a similar fashion, the combination of AsCls and AsBrs forms AsCls Brand AsBrsCl. 

Pentahalides form essentially 1 1 adducts with sulfur, selenium, and tellurium donors, including phosphine sulfides and selenides. IR, Raman (see Raman Spectroscope, and NMR studies of the complexes formed with mixed-donor 0,S ligands showed bonding to occur via sulfur exclusively. The ionic [NbCl4()] -S2R2)][NbCle] complexes are volatile and act as a single-source precursor to mixed-Nb2 05/NbS2 films... 

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