Background Chemistry of Molybdenum
Species in aqueous solutions
Equilibria in aqueous solutions of molybdenum( VI) have been studied in detail . At molybdenum concentrations greater than 10-3 mol l-1 at pH >6 the predominant species is the tetrahedral [MoO4]2- ion. As the pH is lowered polymerisation condensation occurs giving at pH 5-6 the heptamolybdate ion [Mo7O24]6- and at pH 3-5 the octamolybdate ion [Mo8O26]4-. Both ions are built up from linked MoO6 octahedra. At pH 0.9 MoO3 precipitates and in more acidic solutions the [MoO2]2+ ion is formed.
The usual source of molybdenum in the physiological work described later is a molybdate; but it is not always clearly stated in the literature precisely which compound has been used. For example,"ammonium molybdate" may be any of the compounds (NH4)2MoO4, (NH4)2Mo2O7 (dimolybdate), (NH4)6Mo7O24.4H2O (heptamolybdate). The chemical similarities of the various molybdate species and the fact that they are in equilibria in aqueous media mean that major differences are unlikely in their physiological effects. The Table below summarises the aqueous chemistry of molybdenum(VI).
The molybdate species in aqueous solutions depend on the molybdenum concentration and the pH as shown in the Table. Since equilibria are established quickly (i.e., within the time of dissolution or mixing) these are the species whatever the starting compound. In alkaline and neutral solutions molybdates are present as the monomeric [MoO4]2- ion. As the pH is lowered the anion becomes protonated. Whether it polymerises to hepta- or octa- molybdate depends on the pH and the Mo concentration. Polymerisation occurs at higher Mo concentrations. At Mo concentrations above 10-3 mol Mo/l and pH 5 – 6 the heptamolybdate ion forms and at pH 3 – 5 octamolybdate. Note that these are the only polymeric species. The compounds which crystallise from solution under various conditions consist of linked molybdate ions. That we can crystallise a ‘dimolybdate’ does not mean that a [Mo2O7]2- ion is present in solution. At pH 0.9 MoO3 precipitates.
| [Mo(VI)]/mol l-1 |
pH |
Main species |
| All |
> 6 |
[MoO4]2- |
| 10-5 |
> 5 |
[MoO4]2-(ca 100%) |
| 10-5 |
4 |
[MoO4]2- (30%), [HMoO4]- or [MoO(OH)5]- (10%) H2MoO4 or Mo(OH)6 (60%) |
| 10-5 |
2 - 3 |
H2MoO4 or Mo(OH)6 (ca 100%) |
| 10-5 |
1 |
H2MoO4 or Mo(OH)6 (80%) [H3MoO4]+ or [Mo(OH)5(H2O)]+ (20%) |
| < 10-3 |
> 1 |
Monomeric species only |
| > 10-3 |
5-6 |
[Mo7O24]6-, [HMo7O24]5-, [H2Mo7O24]4- |
| > 10-3 |
4 – 5 |
[Mo8O26]4- |
Mitchell, P.C.H., in Ullmann’s Encyclopedia of Industrial Chemistry, 5th Ed., 1990, A16, Chap. 7, pp 675 - 682 and references therein.
Aveston, J., Anacker, E.W. and Johnson, J.S., Inorg. Chem., 1964, 3, 735.
Busey, R.H., and Keller, O.L., J. Chem. Phys., 1964, 41, 215.
In oxidation state V molybdenum is less acidic than in oxidation state VI. Molybdenum(V) oxide, Mo2O5, and hydroxide, MoO(OH)3, are insoluble in neutral and alkaline solutions. Species of molybdenum(V) in hydrogen halide acids have been studied. In concentrated hydrochloric acid the main species is the mononuclear [MoOCl5]2- ion. At lower acid concentrations binuclear [Mo2O3]4+ and [Mo2O4]2+ ions are formed.
Contrary to earlier reports it is now believed that molybdenum(IV) is stable in aqueous solutions and is not subject to disproportionation.
Lamache, M., in Proceedings of the First International Conference on the Chemistry and Uses of Molybdenum, 1973, Mitchell, P. C. H. (ed), Climax Molybdenum Co. Ltd, p. 278.
For molybdenum(III) the existence of the ion [Mo(H2O)6]3+and related dimeric ions has been demonstrated.