Arsenic pentahalide

There is a definite tendency for the nonmetals of the fourth row—As. Se, and Br—to be unstable in their maximum oxidation state. For example, the synthesis of arsenic pentachloride eluded chemists until comparatively recently, v although both PCI, and SbCI, are stable. The only stable arsenic pentahalide is AsFs AsCU decomposes at -50 °C, and AsBrs and Asl5 are still unknown. 

The only known arsenic pentahalides are AsFs and AsCls of which the fluoride is the most stable species. The relative... 

The only arsenic pentahalide definitely known is the fluoride, which is similar to PF5. The action of chlorine on AsF3 at 0° gives a compound whose conductivity in an excess of AsF, suggests—that it—may- be— [AsClJ+ [AsF6]-. 

Numerous adducts, mostly of type MX4L2 (X = Cl, Br, I), are known with nitrogen, phosphorus, arsenic, oxygen, and sulfur donors. They are synthesized by reaction of solid tetrahalides with the ligand or by reduction of pentahalides with the ligand, mainly bidentate or heterocyclic N-donors or phosphines. NbCLt(THF)2 (29) is of particular value since the lability of the THF molecules makes it a useful synthon in Nb chemistry. 

Phosphorus, Arsenic, and Antimony Pentahalides and Aluminum Trichloride... 

The arsenic and antimony pentahalides EX5 (E group 15 element As or Sb X = F or Cl) are strong, irreversible oxidants the gas AsFs has little been used, but SbCF and SbFs are commercially available, very air-sensitive liquids which are used in dry and deoxygenated dichloromethane and liquid sulfur dioxide respectively. SbCls is easier to handle than SbFs which gives the dangerous HF by reaction with moist air. Moreover, SbCls is conveniently used in dichloromethane whereas SbFs is best used in liquid SO2. On the other hand, the side products (halogenation) are more frequently encountered with SbCls than with SbFs. The redox process follows ... 

Elemental arsenic combines with many metals to form arsenides. When heated in the presence of halogens it forms trihalides however, pentahalides with the exception of AsF5 (2) and the unstable AsQ5 are not readily formed. It reacts with sulfur to form the compounds As2S3, AsS, As2S5, and complex... 

Arsenic Halides. Arsenic forms a complete series of trihalides, but arsenic pentafluoride is the only well-known simple pentahalide. All of the arsenic halides, the physical properties of which are given in Table 2, are covalent compounds that hydrolyze in the presence of water. The trihalides form pyramidal molecules similar to the trivalent phosphorus analogues and may be prepared by direct combination of the elements. 

The trichloride and triiodide of arsenic are made by analogous methods. In the case of BiCl3, reaction between the elements may be used or, alternatively, that between SOCl2 and either Bi or Bi203 (HI)- All of these trihalides are hydrolyzed by water, although not so vigorously as the pentahalides. 

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. 

Both trihalides and pentahalides are known for arsenic, antimony, and bismuth. The 12 possible trihalides, all of... 

The decreasing stabUity of the +5 oxidation state is shown by the fact that all 16 EX3 compounds (E = P, As, Sb, Bi X = F, Cl, Br, I) are formed, but only phosphorus forms pentahalides with all four halogens. Arsenic and antimony form only the pentafluoride and pentachloride, whereas bismuth forms only the pentafluoride. Of the pentafluorides of the lower three pnicogens, only ASF5 is trigonal bipyrami-dal. SbFj and BiFj are polymers of EF octahedra held together by bridging fluorine atoms. 

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