Molybdenum heteropoly anions

Mild acidification of molybdate-phosphate solutions yielded the colorless [P2Mos023]"6 anion, isolated as the sodium salt48,49 The heteropoly acid containing pentavalent antimony, H3[SbMo12O40] 48 H20, was reported to have been prepared by refluxing M/20 molybdic acid with M/75 potassium pyroantimonates°), however, more work is necessary to elucidate to exact nature of molybdenum heteropoly compounds containing antimony. 

Molybdenum is of intermediate stability compared with heteropoly anions of other elements. With 12-heteropolytungstates and 12-heteropolyvanadates, the stabilities follow the order W > Mo > V. 

Another class of 12-heteropoly anions involves the replacement of molybdenum by pentavalent vanadium. The molybdovanadophosphoric acids,... 

Niobium, like vanadium, can replace molybdenum in the heteropoly anions. The yellow 10-molybdo-2-niobophosphate anion has been prepared104). The salts (C5HsNH)4H[PMoioNb204o] and [(CH3)4N]4H[PMoioNb2040] have been prepared and characterized. The free acid could not be isolated since it is unstable even in solution104). 

In addition to the traditional iso- and heteropoly anions just discussed, recent work has revealed that others, many of them very large, with crown-type structures can be obtained. Examples of these are the [NaPsW Ono]14- ion, which has a double crown structure with five-fold symmetry, [H7P8W4gOi84]33 , which has a double crown structure with four-fold symmetry and two structurally similar crown species of molybdenum, [Mo36On2]8 , and Mo34Oiio(NO)4(H20)14. ... 

In the structure of isopoly and heteropoly anions the tungsten and molybdenum atoms lie at the centres of octahedra of oxygen atoms and the structures are built up of these octahedra by means of shared corners and shared edges (but not shared faces). 

Since oxidation states difficult to study in aqueous solutions, such as manganese(IV) and nickel(IV), form heteropoly-anions with molybdenum and tungsten (Pope 1983) one may hope that Pr(IV) might occur in solution in such complexes. The cerium(III) complex CeWjgOj/ has E"=1.1V, showing that cerium(IV) is more stabilized in this environment than by sulfate, but still not to any extreme extent. Actually, Pr(PWu039)j" is known. We return below to the related question of Tb(IV) in solution. 

So, H. and Pope, M. (1972). Origin of Some Charge-Transfer Spectra. Oxo Compounds of Vanadium, Molybdenum, Tungsten, and Niobium Including Heteropoly Anions and Heteropoly Blues,/no/j. Chem., 11, pp. 1441—1443. 

SiMouO ]4-.134 The structure of the heteropoly blue species (four-electron-reduced PM012O40]3-) formed in the molybdenum blue determination of phosphorus has been reported135 and a review of molybdate heteropoly blues has appeared.130 A stability index of polyanion structures has been discussed.137 The presence of two PMo, anions in aqueous solutions of molybdate and phosphate has been demonstrated by 31P NMR.138 Solid state NMR (nonspinning and MAS techniques) has been used for characterization of heteropolyanions.139 Molybdenum-95 NMR spectra of some polymolybdates have been reported.140... 

Heteropoly compounds may be classified according to the ratio of the number of central atoms to the peripheral molybdenum or other such atoms. Compounds with the same number of atoms in the anion usually are isomoiphous and have similar chemical properties. Usually, the heteropolymolybdates and heteropolytungstates containing nontransition elements as central atoms have more structural analogues than those that contain transition elements as central atoms. Table 1 lists all elements... 

The 6-heteropoly compounds of molybdenum have been studied extensively and are well understood. Typical examples are the 6-molybdotellurate anion, [TeMo6024]-6, and the 6-molybdochromate(III) anion, [CrMo6024H6]-3. These are usually prepared by the reaction of ammonium paramolybdate with a soluble salt of the central atom7. Some of these are stable only in very mild acid solution and caution should thus be exercised in preparing them. 

The classic poly adds of molybdenum and tungsten are of two types (a) the isopoly acids and their related anions, which contain only molybdenum or tungsten along with oxygen and hydrogen, and (b) the heteropoly acids and anions, which... 

In addition to the species consisting of only molybdenum or only tungsten, there are mixed anions, of both iso- and heteropoly types in which the metal sites can be occupied by both molybdenum and tungsten atoms.8 Partial substitution by vanadium atoms or other Group 5 (e.g., Nb) atoms can also occur.9... 

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