Redox potentials have been reported for molybdenum in aqueous solutions of mineral acids [Latimer, 1952; Bard et al. ,1985]. The species have molybdenum in coordination with oxide, hydroxide, water, sulfate, and chloride. The relative stabilities of different oxidation states with respect to oxidation and reduction depend on their coordination environments [Mitchell, 1990 and references therein].
W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions , Prentice-Hall, New York, 2nd Edn., 1952, p. 245.
Bard, A. J., R. Parson and J. Jordan: Standard Potentials in Aqueous Solutions, Marcel Dekker, New York, 1985, p. 480.
Mitchell, P.C.H., in Ullmann’s Encyclopedia of Industrial Chemistry, 5th Ed., 1990, A16, Chap. 7, pp 675 - 682 and references therein.
In acidic solutions molybdates are much less powerful oxidising agents than vanadates and chromates. With increasing pH the oxidation potential decreases. In mineral acid solutions molybdenum(VI) is reduced to molybdenum(V) by mild reducing agents (tin(II), mercury metal) and to molybdenum(III) by stronger reducing agents (zinc amalgam).
Oxidation states below III are not obtained by reduction of molybdenum aqua- and oxo-species nor is the IV oxidation state. In neutral and alkaline solutions molybdates ( Mo(VI)) may be reduced by dithionite (often regarded as the in vitro analogue of biological reducing agents) to molybdenum(V) but weaker reducing agents including hydrogen sulfide and thiols give molybdenum blue. Aqueous solutions of molybdenum(III) oxidise in air to molybdenum(V) and (VI).
There are a number of observations which indicate how different ligands stabilise different oxidation states. In neutral and alkaline solutions in the presence of the sulfur-donor ligands, dithiophosphate, dithiocarbamate, and xanthate, molybdenum(VI) is reduced by dithionite ultimately to oxomolybdenum(IV) complexes. This observation is of particular interest in view of suggestions that molybdenum( IV) is formed in the reduction of the enzyme xanthine oxidase. It is also of interest that the five-coordinate oxomolybdenum( IV) sulfur complexes will abstract an oxygen atom from various organo-oxygen compounds, e.g. pyridine-N-oxide [Holm,1987]. With cyanide in basic solution molybdenum(VI) generally undergoes reduction to the very stable octacyanomolybdate(IV) ion. However, reduction with hydrogen sulfide in the presence of cyanide gives a molybdenum( II) complex ion, [Mo(CN)7]5-, a rare example of reduction below the 3+ state in aqueous solution. [Drew et al., 1976].
Holm, R. H., Chem. Rev., 1987, 87 ,1401.
Drew, M. G. B., Mitchell, P. C. H. and Pygall, C. F., J. Chem. Soc. ( Dalton ) , 1976, 1071.
In considering the relative stabilities of molybdenum oxidation states in aqueous solutions it should always be remembered that many molybdenum complexes readily hydrolyse and so the species undergoing redox reactions may be oxo- or hydroxo- species.
Redox reactions of oxomolybdenum species involve changes in the number of oxide or hydroxide ions coordinated to molybdenum and so it is reasonable that complexes of this type will be involved in reactions in which oxygen atoms or ions are transferred. Such reactions include the oxidation (hydroxylation) of xanthine to uric acid and reduction of nitrate to nitrite catalysed by molybdenum enzymes. It should be noted that electron transfers do not always take place at the metal atom in a complex. In a series of molybdenum complexes with dithiol ligands [Mo(SS)3], [Mo(SS)3]- and [Mo(SS)3]2- it is considered that the oxidation state of molybdenum remains constant at 4+ and that electron gain or loss occurs in orbitals located mainly on the ligands.
Concouvanis, D., Progr. Inorg. Chem. , 1970, 11, 234.
Chen, J., Christiansen, J., Campbasso, N., Bolin, J. T., Tittsworth, B. C., Hales, B. J., Rehr, J.J. and Cramer, S. P., Angew. Chem. Int. Edn. , 1993, 32, 1592.