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Oxidation states

In its compounds molybdenum exhibits all oxidation states from -II to V1. The lowest oxidation states, -II to I, occur in complexes with π-acceptor ligands, mainly carbon monoxide, cyclopentadiene, and related compounds, nitric oxide, and phosphorus- and arsenic-donor ligands. The best known compounds of molybdenum(II) are those containing the polynuclear ion [Mo6Cl8]4+ Binuclear carboxylates, e.g. [Mo2(acetate)4] , are also known. In these complexes molybdenum(II) is stabilised by formation of metal-metal bonds. Mononuclear complexes of p-acceptor ligands with six- and seven-coordinate molybdenum(II) have been prepared.

The low oxidation states of molybdenum(-II to II) occur in coordination environments which are unlikely to be encountered in biological systems and so are not expected to arise in enzymatic processes.

In oxidation states III to VI molybdenum forms a large number of complexes with oxygen- and nitrogen-donor ligands and with the halogens. Complexes with sulfur-donor ligands are common but there are few complexes with phosphorus- and arsenic-donor ligands. There is an extensive aqueous chemistry of these oxidation states.

Molybdenum(IV) is strongly stabilised by cyanide in the complex ion [Mo(CN)8]4-. There are also indications that molybdenum(IV) may be stabilised by sulfur: the disulfide is the most stable sulfide.

The chemistry of the V and VI oxidation states is dominated by oxomolybdenum species with one or more oxygen atoms as terminal ligands (i.e., bonded to only one molybdenum atom) or as bridging atoms (i.e., bonded to two molybdenum atoms). An important feature is p-donation from oxygen to the metal giving a strong multiple bond. This type of donor-p-bonding occurs less readily as the formal oxidation state of the metal decreases and so oxo-species of molybdenum(IV) are less common and species with terminal oxide do not occur for molybdenum(III), (although species with bridging oxide and hydroxide are known). The point is illustrated by a series of isothiocyanato complexes.

Isothiocyanato complexes of molybdenum
[MoO(NCS)5]2- [Mo(NCS)6]2- [Mo(NCS)6]3-

With certain sulfur-donor ligands mononuclear oxomolybdenum(lV) complexes are known. In these sulfur complexes molybdenum(IV) is five-coordinate.The complexes with diethyldithiocarbamate provide a series of molybdenum complexes in oxidation states VI to III:

Dithiocarbamato complexes of molybdenum
[MoO2(S2CNEt2)2] [Mo2O3(S2CNEt2)4]
Mo(IV) Mo(III)
[MoO(S2CNEt2)2] [Mo2(S2CNEt2)6]

The existence of such a series of complexes is of particular interest in view of the involvement of molybdenum with sulfur ligands in molybdenum containing enzymes.

Mitchell, P. C. H., J. Inorg. Biochemistry , 1986, 28 , 107
Holm, R. H., Chem. Rev., 1987, 87 ,1401.

Compounds of molybdenum(V) and (Vl) which are not derived from oxo-species are rare. Examples are the hexa- and penta-halides which are, however, readily hydrolysed.

Redox behaviour and the relative stability of oxidation states

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.

Molybdenum in chemical evolution: stepwise oxygenation of the Proterozoic ocean ― molybdate as a marker

Oxygenation of the Earth's atmosphere is thought to have proceeded in two broad steps near the beginning of the Proterozoic eon (2,500 million years ago) and its end (542 million years ago). The oxidation state of the Proterozoic ocean between its beginning and its end and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth. A new perspective on ocean oxygenation based on the authigenic accumulation of molybdenum in sulfidic black shales is presented.

By 2,650 Myr ago accumulation of authigenic molybdenum from sea water is already seen in shales. The small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents.

At roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen, in deposited shales enrichments appear which are indicative of persistent and vigorous oxidative weathering.

After about 1,800 Myr ago expansion of sulfidic conditions maintained a mid- Proterozoic molybdenum reservoir at below 20 per cent of the modern concentration, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic ( sulfidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms(10).

By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulfidic conditions in the sediments and water column.

Scott, C., Lyons, T. W., Bekker, A., Shen, Y., Poulton, S. W., Chu, X., and Anbar, A. D., Tracing the, Nature, 2008, 452, 456-4U5.

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