Heteropoly-Acids with Vanadium

Vanadium possesses the property of entering into the composition of a large number of compounds which contain it as a constituent of a complex anion. Light has been thrown on the nature and constitution of these compounds by investigations into other heteropoly-acids, which are now found to be most satisfactorily formulated by the application of a modification of Werner's co-ordination theory first suggested by Miolati, and extended by Rosenheim and his co-workers. According to this theory the heteropoly-acids are produced by hydration of acids having the general formula H_8-n[RⁿO₄], where n represents the valency of the metal R. Thus, for example, in the cases of silicon and phosphorus the following schemes obtain:

H₄[SiO₄] + 2H₂O = H₈[SiO₆].
H₃[PO₄] + 2H₂O = H₇[PO₆].

Or, generally,

H_8-n[RⁿO₄] + 2H₂O = H_12-n[RⁿO₆].

It will be observed that the basicity of each of these acids is determined by deducting the valency of the central atom in the complex anion from 12. The greatest number of oxygen atoms or other divalent radicals that can be co-ordinated with the central atom is 6; and this number usually holds good in a true compound, but not always. For example, the co-ordination number of the acid is only 4.

Substitution of the co-ordinated oxygen atoms by divalent radicals yields the various heteropoly-acids. The vanado-phosphates, for instance, axe derivatives of the hypothetical phosphato-acid H₇[PO₆], in which one or more of the oxygen atoms have been replaced by divalent (V₂O₆)'' radicals. The compound to which the formula 7(NH₄)₂O.P₂O₅.12V₂O₅.26H₂O has been ascribed from its analytical data thus becomes (NH₄)₇[P(V₂O₆)₆].13H₂O. If some of the oxygen atoms in the complex anion are replaced by (V₂O₆)'' radicals and others by (Mo₂O₇)'' radicals, the molybdovanado-phospliates are produced, e.g. and . Similarly, substitution by both (V₂O₆)'' and (W₂O₇)'' radicals gives rise to the tungstovanado-phosphates. The heteropolyvanado-arsenates are derived from the hypothetical arseno-acid H₇[AsO₆]. It has recently been shown that the arsenic in these compounds may undergo partial substitution by phosphorus to give rise to series of mixed crystals which are called heteropolyvanado-arseno phosphates or heteropolyvanado - phosphoarsenates. The heteropolyvanado-silicates are derived from the acid H₈[SiO₆].

The existence of such associated radicals as (V₂O₆)'', (Mo₂O₇)'', and (W₂O₇)'' has been assumed in order to explain the very frequent occurrence of twelve (½V₂O₅), MoO₃, or WO₃ radicals in the complex anions of compounds the co-ordination number of which is six. A greater number than twelve has never been observed. As a rule only four of the oxygen atoms can undergo substitution by these associated radicals; thereafter an isomeric change appears to take place, thus:



Compounds are said to be saturated when all the oxygen atoms of the parent anion are replaced by metallic acid anions; unsaturated compounds contain some replaceable oxygen.

The decision as to which element forms the central atom of the complex anion depends on the fact that in any particular series of compounds the atomic proportions of the metals in the place of the co-ordinated oxygen will show very considerable variation when calculated for one atomic weight of the element which constitutes the central atom. Among the molybdo-vanadophosphates many are known which contain varying amounts of molybdenum and vanadium in combination with one gram-atom of phosphorus. Hydrogen also provides the nuclear atoms in some series, which are best viewed as derivatives of a hypothetical co-ordinated hexa-aquo-acid, H₁₀[H₂O₆]. The vanado-tungstates (or tungsto-vanadates) are, for instance, represented by the general formula . Compounds of this type contain water of constitution.

It should be noted that vanadium does not as a rule form the central atom of the complex anion in the heteropoly-compounds. The oxalo-vanadates, however, most probably contain the anion , which is obtained by substituting two oxygen atoms in the anion of orthovanadic acid, i.e. the [VO₄]''' ion by two (C₂O₄)'' groups. Substitution by (MoO₄)'' groups gives rise to the oxalo-molybdovanadates of the general formula:

or .

Uranyl-vanadates, which are probably derived from uranyl-vanadic acid, , have also recently been prepared.

The constitution of many of the heteropoly-acids and of their salts may thus be explained, although direct proof of the correctness of some of these formulae is lacking. Some of the heteropoly-compounds of vanadium are, however, very complex, and cannot be represented by application of the foregoing scheme. To take an extreme instance, Rogers prepared a black, crystalline compound to which he ascribed the following formula:

99(NH₄)₂O.12P₂O₅.2As₂O₅.66V₂O₅.6V₂O₃.191WO₃.522H₂O.

The composition of these substances is necessarily a matter of doubt, because of the difficulty of conducting exact analyses of compounds of very high molecular weight; a small difference in the analytical data makes a large difference in the number of molecules. Further, the heteropoly-compounds are characterised by the ease with which they undergo hydrolysis; even the process of recrystallisation frequently produces a different complex.

In many cases the salts of any one series which have been analysed are not true compounds, but consist of isomorphous mixtures of simple chemical compounds; the composition of the mixtures varies with the composition, temperature, and acidity of the media from which they separate. This has been shown to apply to the molybdo-vanadates (or vanado-molybdates), molybdo-vanado phosphates, vanado-selenous acid and the vanado-selenites as well as to telluric acid and its salts. Recently, Canneri has succeeded in preparing a large number of mixed crystals of salts, and in some cases of the free acids, which belong to different series; for example, mixed crystals of tungsto-vanadophosphates and tungsto-vanadoar senates, of tungsto-vanado-phosphates and molybdo-vanadophosphates, of tungsto-vanadoar senates and molybdo-vanado arsenates; and, finally, mixed crystals of members of all the four series mentioned have been obtained. The mixed crystals have the same crystalline habit as that of their components, and complete isomorphism exists between any two series which mix in all proportions to form continuous series of mixed crystals containing four or five different oxygenated acid radicals. The crystallographic data of a number of ammonium tungsto-vanadoar senates have also recently been independently determined, and it has been shown that the crystal form is unaffected when the arsenic is substituted by phosphorus, or when the (W₂O₇)'' radicals are partially substituted by (Mo₂O₇)''.

It appears probable that adsorption of uncombined radicals by molecules which contain a large number of these radicals, but which are not saturated, takes place, and that the only true chemical individuals are the maximum co-ordinated compounds. It has been shown that vanadium pentoxide forms a colloidal solution, in which state it is readily adsorbed.