Paramolybdate ion

Fig. 5. Structure of the paramolybdate ion IM07O24I 6-Here seven MoOg octahedra share edges to make the complete unit...

The subsequent steps must be more complicated and as the pH of the solution is lowered to about pH 6 polymerization is detectable. It was proposed by Lindqvist, primarily because of its existence in crystals, that the first main reaction at this stage is to form the paramolybdate ion ... 

The so-called dimolybdates, such as Na20-2Mo03-nH20, which sometimes crystallize from solutions in the pH range of 5-6, are probably mixtures of normal molybdates and paramolybdates. The various trimolybdates of general formula M20-3Mo03- H20 appear to contain acid paramolybdate ions or other ions with seven or more Mo atoms. 

Note Commercial ammonium molybdate has the formula (NH4)6Mo7024. 4H20 (a paramolybdate) and not (NH4)2Mo04 but the ion Mo04 is employed in the equations for purposes of simplicity. These ions may exist under the experimental conditions of the reaction. 

In addition to heteropolyacids there are isopolyacids, the commonest of which are the parent acids of the paramolybdates, Hg[Mo7024], and of the paratungstates, H5[HWg02i]. The structure of the [Mo7024] ion (Lindqvist, 1950) is like that of the 6-acid anions (above) but with a central, octahedrally co-ordinated Mo atom. 

Tetramolybdates or metamolybdates, M20-4Mo03- H20, are formed from concentrated solutions of alkali molybdates treated with 1.5 mols per mol of hydrochloric acid. Octamolybdates are obtained similarly from solutions containing 1.75 mols per mol of hydrochloric acid or by adding the calculated amount of M0O3 to ammonium paramolybdate solution. Both the tetra- and octa-molybdates are derived from the ion [Mo8026]4. ... 

The rate constants show some dependence on the nature of the cation present from the added MOH. This is ascribed to significant ion-association, which is not too surprising with a 6— anion, albeit a relatively large one. At a pH between 9 and 10, and at an ionic strength of 2.5 mol 1 (NaCl), the activation parameters are Aff = 28.3 0.5 kcal mol and AS" = -I-17 2 cal deg mol". There is a marked similarity of the kinetic pattern here to that established for the paramolybdate ([Mo7024] ) anion. ... 

The multi-component LDHs (precursors) were prepared by reacting magnesium and aluminum nitrate solutions with a potassium hydroxide + potassium carbonate solution followed by anionic exchange to incorporate of decavanadate-, paramolybdate-, oxoniobate- and oxotungstate-iones into LDHs accordingly to experimental procedures... 

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