Molybdates, isopoly

Normal isopoly- and peroxymolydates of ammonium and several metals are known. The normal or orthomolybdates may be considered as salts of molybdic acid having formulas H2Mo04 xH20 or M20 Mo03 xH20. They are either of monoclinic or scheelite type crystal structure and obtained as hydrated salts. 

Salts of Molybdic and Tungstic Acids. What types of salts of molybdic and tungstic acids exist Give examples of isopoly- and heteropoly molybdenum and tungsten compounds. What is the composition of para- and metamolybdates and of para- and metatungstates Which molybdates and tungstates dissolve poorly in water ... 

A substantial number of Mo peroxo complexes has been structurally characterized. These complexes all have nearly symmetrically side-on-bound peroxide. Among them are mononuclear, dinuclear, tetranuclear and heptanuclear compounds. The tetranuclear and hep-tanuclear complexes are related to the isopoly molybdates. Structural studies have identified... 

Figure 18-C-3 Some of the important isopoly anion structures. Structures known only for molybdates or tungstates are [HjWjjC ]10 shown at top and bottom.

These are dark blue species obtained by reduction of both isopoly- and heteropoly molybdates and tungstates, and thus the class is broader than the name implies. It appears that in general, these reductions are reversible and major structural features are retained. In the case of the [M6Oi9]2 ions, reduction by one electron gives a 3-ion in which the added electron is weakly trapped at one metal atom but is thermally mobile. 

Fig. 26-C-2. (a) Diagrammatic representation of Mo06 and W06 octahedra used in showing structures of some isopoly and heteropoly anions, (b) The structure of the para-molybdate anion, [M07O24]6". (c) The structure of the octamolybdate anion, [Moa026l4 (note that one Mo06 octahedron is completely hidden by the seven that are shown), (d) The structure of the [W12C>4.2]12- unit in the paratungstate ion.

Derivatives of the weaker, oxygen-containing metallic acids, such as those of tungstic, molybdic or vanadic acids, exhibit a quite characteristic behavior, and may therefore be considered as the classical isopoly compounds. One property characteristic of these metallic acids is the more or less sharply pronounced hydrolysis of their salts in aqueous solution, particularly in the presence of H+ ions. The hydrolysis products then undergo, over a period of time, a secondary reaction, combining to more highly aggregated ions, that is, the isopolyanions. For example ... 

Heteropoly compounds are composed not only of the weak, oxygen-containing metallic acids (tungstic, molybdic and vanadic), but also of moderately strong to weak acids of nonmetals, e.g., boric, silicic, phosphoric, arsenic, telluric, etc., acids. Stable heteropoly compounds very frequently show nonmetallic to metallic acid ratios of 1 12, 1 6 or 1 9. Since the heteropoly compounds form under conditions similar to those in which isopoly compounds are obtained, that is, only in solutions containing H+ ions, it is assumed that the building blocks of the heteropolyanions are isopoly anions [1]. 

The nature of molybdic acid in water and in acidified solutions has been studied by relatively few workers, JONES (1) proposed the existence of isopoly-cations of molybdenum to explain the solubility of molybdic acid in acid solutions. More recently, AVESTON, ANACKER, and JOHNSON (2) report ultracentrifuge studies of molybdic acid in hydrochloric acid medium. They find a degree of polymerization of about 3 in 0,8 M HCl, but note that equilibrium is not attained. On the other hand they find no aggregation and apparent equilibrium in 6 M HCl, No quantitative results were reported of experiments in HCIO medium. 

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