Halides with Oxidation States

Halides of the type AX6, where A is P, As, Sb, or Bi, and X is F, Cl, Br, or I, may generally be obtained by interaction of the elements. It is also possible to react AX3-type compounds with the corresponding halogen, and this is particularly useful when X = F. The halides SbF6 and BiF6 are made in this way. The compounds SbBr6 (cf. Section II,A,2), Sbl6, and arsenic(V) and bismuth(V) halides other than the fluorides are unknown. 

Exchange reactions may be used in preparing PF6 (178), AsF6 (115), and SbF6 (95, 165)  

All halides of the type AX6 are very hygroscopic and are hydrolyzed by water. Bismuth pentafluoride, a white solid, becomes yellow with the least trace of water and readily fluorinates other substances, with reversion to BiF3. 

In this compound, the C—P—C angle is probably intermediate between 90°, the value for the equatorial-axial position angle, and 120°, the ideal value for the equatorial-equatorial position angle, and this makes the exchange possible. In accordance with this hypothesis, the cyclic compound (CHOsPFs shows no exchange in this case the C—P—C angle must be almost 120°. 

The structure is different in the solid state, and X-ray investigations show that the lattice of the solid P(V) chloride is built up of [PC14]+ and [PC16] ions (27). This is confirmed by the Raman spectrum (52). Many AX6-type compounds are found to be capable of existing in two different forms, one of which is molecular and the other saltlike. Thus, solid PBr6 contains the ions [PBr4]+ and [Br] (152). In nonpolar solvents such as 

---

When an element has more than one oxidation state the lower halides tend to be ionic whilst the higher ones are covalent—the anhydrous chlorides of lead are a good example, for whilst leadfll) chloride, PbCl2, is a white non-volatile solid, soluble in water without hydrolysis, leadflV) chloride, PbC, is a liquid at room temperature (p. 200) and is immediately hydrolysed. This change of bonding with oxidation state follows from the rules given on p.49... 

There are no convincing reports of halides in oxidation states below III early reports of Osl and OsI2 seem to result from oxide contaminations. Neither is there OSF3, evidence of the greater stability of the +4 state compared with that of ruthenium. 

The obtention of nitrides by thermal decomposition of the azides was only successful for the synthesis of [MNBr4]2- from [MN3Br5] . Thermolysis of MX5 with ammonium salts afforded a more general access to nitride halides in oxidation state V, or IV if higher temperatures were used.316,317 Oxo nitrides were obtained if moisture was admitted during the process.318... 

In addition to oxides and halides, high oxidation states are sometimes found with surprising ligands, such as in the ion [ReH9]2- (1), which is formally a hydride complex of Re(VII). 

Metastable solutions of the monohalides AIX and GaX (X = Cl, Br, I) have been prepared using this technique, and oligomeric species (MX) -Em containing a variety of donors (E) have been crystallised. The solutions disproportionate to the trihalide and the metal (equation 2) when warmed to temperatures in the range -40 to 4-50 °C, depending on the halide, the donor, and the concentration. Species with oxidation states both higher and lower than +1 (i.e. on the path to both disproportionation products) have been isolated. The reduced species (0 < Nox < 1) are discussed in the section on metalloid clusters, and the monohahdes and more oxidized species (1 < Nox < 3) are discussed here. A sonochemical synthesis of a subvalent galhum species, possibly Gal, has also been developed. ... 

The reduction of rare-earth metal halides with unlike metals, Wohler s metallothermic reduction (Wohler, 1828), has originally been used to produce the rare-earth metals (Klemm and Bommer, 1937). When used properly, intermediates with oxidation states between +3 and 0 can be obtained. 

Anhydrous halides, however, are obtained when the metal is heated with the dry hydrogen halide or the halogen. In the case of elements with more than one oxidation state, the hydrogen halide produces a lower halide and the halogen a higher halide, for example... 

As already noted, the simple salts in this oxidation state are powerful oxidising agents and oxidise water. Since, also, Co(III) would oxidise any halide except fluoride to halogen, the only simple halide salt is C0F3. Cobalt(lll) Jluoride, obtained by reaction of fluorine with cobalt(II) fluoride it is a useful fluorinating agent. 

Phosphoms shows a range of oxidation states from —3 to +5 by virtue of its electronic configuration. Elemental P is oxidized easily by nonmetals such as oxygen, sulfur, and halides to form compounds such as 2 5 2 5 reduced upon reaction with metals to generate phosphides. The... 

Rhenium Halides and Halide Complexes. Rhenium reacts with chlorine at ca 600°C to produce rheniumpentachloride [39368-69-9], Re2Cl2Q, a volatile species that is dimeric via bridging hahde groups. Rhenium reacts with elemental bromine in a similar fashion, but the metal is unreactive toward iodine. The compounds ReCl, ReBr [36753-03-4], and Rel [59301-47-2] can be prepared by careful evaporation of a solution of HReO and HX. Substantiation in a modem laboratory would be desirable. Lower oxidation state hahdes (Re X ) are also prepared from the pentavalent or tetravalent compounds by thermal decomposition or chemical reduction. 

A chain mechanism is proposed for this reaction. The first step is oxidation of a carboxylate ion coordinated to Pb(IV), with formation of alkyl radical, carbon dioxide, and Pb(III). The alkyl radical then abstracts halogen from a Pb(IV) complex, generating a Pb(IIl) species that decomposes to Pb(II) and an alkyl radical. This alkyl radical can continue the chain process. The step involving abstraction of halide from a complex with a change in metal-ion oxidation state is a ligand-transfer type reaction. 

---