Group 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. 

These considerations are well illustrated by looking at the Group 15 pentahalides. The bonding in the trigonal bipyramidal molecules has been discussed in Chapter 6, and the postulate of nd orbital participation helps to rationalise their occurrence. No nitrogen(V) halides are known, although NF5 may have been detected as a short-lived species at low temperatures. The cation NF4 occurs in solids, prepared by reactions such as ... 

Their hydrides MH3, and trihalides are pyramidal but pentahalides are trigonal bipyramidal Group-16 Elements Chalcogens Increasing Decreasing Exceptions Trends Trends... 

Group-5 elements are most stable in their maximum oxidation state +5 and therefore form pentahalides, see Figure 7. Most volatile are the pentafluorides, followed by the pentachlorides and the pentabromides. Besides the pure halides, also the oxyhalides (MOX3) are stable in the gas phase. They should be less volatile compared to the pure halides. This was confirmed experimentally for niobium, see Figure 8. 

The pentahalides of Group VA are all strong Lewis acids that interact with electron pair donors to form complexes. This is typified by the formation of halo complexes,... 

There is a strong tendency of the pentahalides of the heavier members of Group VA to form complexes. The interaction of SbCl5 and SbCl3 shows that there is a complex formed between the two molecules,... 

Various halo-alkyl carboxylates have been reported, examples are TaCl(OAc)4 and [MC14(02CR)]. The latter contain dinuclear molecules with syn—syn bridging carboxylate groups (18-B-II) and are prepared from the pentahalides and the corresponding acid41 as in ... 

They are obtained by reduction of the pentahalides by the metal alone, or in the presence of sodium chloride. Their key structural unit, an [M6X12]"+ group, is shown in Fig. 18-B-ll. A typical reaction is... 

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 ... 

The 40-electron group. The only complexes in this group with the full symmetry of the trigonal bipyramid are Group V pentahalides and the SnCls ion. In SOF4 the 0 atom occupies an equatorial position and the structure is necessarily somewhat less symmetrical. It is interesting that the InCls ion, isoelectronic with SnCls, has... 

Na2C03.10 H2O. Here the Na(H20)g groups are associated in pairs to form units Na2(H2 0)io of the same general type as the dimers of certain pentahalides. Contrast the structure of Na2S04.10 H2O above. 

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. 

Phosphorus, like nitrogen, has five valence electrons. These are in the third quantum shell, in which there are d orbitals in addition to s and p orbitals. Phosphorus forms up to six separate bonds with other atoms, but covalencies greater than four are usually exhibited only in combination with halogens (in the pentahalides and PX ions) and with groups such as phenyl, CgHs. (For an example of 5 coordination by 0 see p. 684.) The d orbitals are used for a bonding if more than four bonds are formed, but they are also used for vr bonding in tetrahedral oxy-ions and molecules. A very simple summary of the stereochemistry of P may be... 

Pentahalides and molecules MX5 valence group 10 The following pentahalides are known ... 

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... 

BTF is prepared industrially from toluene in two synthetic steps 1) free radical perchlorination of the methyl group, followed by 2) fluorine/chlorine exchange of the three chlorine atoms with anhydrous hydrogen fluoride (Scheme 1). The chlorination step may be catalyzed by light of suitable wavelength (UV) and is conveniently carried out in the liquid phase. The fluoride/chloride exchange can be catalyzed by the presence of metal halide compounds, such as pentahalide (Cl, F) salts of antimony and molybdenum, and is effected under a variety of... 

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