Pentahalides

Apart from the trigonal bipyramidal SbClj molecule, which has the same structure in the crystalline as in the vapour state, we have to deal here with 6- and 7-coordinated structures. 

The octahedral structures, described in Chapter 5, include the dimeric and tetrameric molecules of types (a) and (b) (Fig. 9.12) and the c/s and trans chains. 

The special interest of the two M4X20 structures lies in the fact that the —F— bond angles are quite different, 180° (as shown at (b)) and 132° in the c.c.p. and 

The two forms of octahedral MXs chain arise by sharing of adjacent or opposite vertices of octahedral MXg groups cis and trans chains) here again it would be interesting to know what determines the choice of chain. 

Of the 7-coordinated structures, that of (3-UF 5 is a 3D structure in which U is surrounded by 7 F of which 4 are shared with other coordination groups (Fig. 28.2). In the Pads structure (which is unique to that compound) pentagonal bipyramidal groups share two edges to form infinite chains (Fig. 9.13). Bond lengths are Pa—Cl (bridge), 2-73 A, Pa—Cl (terminal), 2-44 A. 

---

The pentahalides of phosphorus, PX, in the gas phase exhibit varying tendencies to dissociate into trihaUde and halogen. InstabiUty increases with increasing ionic radius of the halogen. The pentafluoride appears to be thermally stable. Dissociation of the pentachloride, a few percent at 100°C and 101.3 kPa (1 atm), is essentially completed at 300°C (36). The pentabromide is partially dissociated in the Hquid state and totally dissociated above ca 35°C (39). Pentaiodide does not exist. The molecules of PF and PCl in the vapor phase are trigonal bipyramids. In the crystalline state, both pentachloride and pentabromide have ionic stmctures, ie, [PClJ IPClg] and [PBr4]" PBrJ , respectively. The PX" 4 cations are tetrahedral and the PX anion is octahedral (36,37). 

Until fairly recently only the pentafluorides and SbCE were known, but the exceedingly elusive AsCE was finally prepared in 1976 by ultraviolet irradiation of AsCE in liquid CE at — 105°C. Some properties of the 5 pentahalides are given in Table 13.9. 

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... 

However, most complexes of Nb and Ta are derived from the pentahalides. NbFs and TaFs dissolve in aqueous solutions of HF to give [MOFs] " and, if the concentration of HF is increased, [MFg]. This is normally the highest coordination number attained in solution though some [NbFy] - may form, and [TaFv] " definitely does form, in very high concentrations of HF. However, by suitably regulating the concentration of metal, fluoride ion and HF, octahedral... 

M(SCH2CH2S)3] with stereochemistry midway between octahedral and trigonal-prismatic, are known for both Nb and Ta. The pentahalides of these two metals act as Lewis acids and form complexes of the type MX5L with O, S, N, P. and As donor ligands. 

Until comparatively recently only vanadium had a significant coordination chemistry and even so the majority of its compounds are easily oxidized and must be prepared with air rigorously excluded. The usual methods are to use VCI3 as the starting material, or to reduce solutions of vanadium(V) or (IV) electrolytically. However, the reduction of pentahalides of Nb and Ta by Na amalgam or Mg, has facilitated the expansion of Nb " and Ta " chemistry particularly with S-and P-donor ligands. 

Table 28.3 is a list of the known halides only gold forms a pentahalide and trihalides and, with... 

Two octahedra sharing one edge correspond to the composition (MX5)2 or (MX X ) This is the kind of structure common among pentahalides and ions [MX5]2 when X = Cl, Br or I ... 

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. 

The pentahalides of group VA elements are strong Lewis acids that react readily with electron pair donors such as halide ions to form complexes. 

---