Higher oxidation states

The commonly encountered compounds of molybdenum in its applications are molybdenum trioxide and molybdates (oxidation state VI) and molybdenum disulfide (oxidation state IV) (see Table: Some Uses of Molybdenum Compounds).

Oxomolybdenum species
The chemistry of molybdenum in its higher oxidation states is dominated by oxo-species which may have terminal oxide (1) or bridging oxide (2) or both.

A feature of the oxo-species is multiple bonding from oxide to molybdenum which gives rise to a strong characteristic infrared absorption at 900-1000 cm -1.

Oxomolybdenum species

Fig. 1: Oxomolybdenum species

We may think of the formation of the oxo-species as a consequence of the polarisation of water molecules by the highly charged cations leading to the release of protons and the formation of multiple bonds to oxygen, e.g.:

[Mo(H2O)66+ + 6H2O  arrow formula [MoO4] 2- + 8H3O +

Molybdates and polymolybdates
These are compounds of molybdenum(VI): sodium molybdate, Na2MoO4; ammonium dimolybdate, (NH4) 2Mo2O7, ammonium heptamolybdate, (NH4)6MO7O24.4H2O and ammonium octamolybdate, (NH4)4Mo8O26. They are readily available commercially and are the starting point for the preparation of many other molybdenum compounds.

Sodium molybdate crystals contain the discrete tetrahedral [MoO4] 2- ion. The structures of the polymolybdates consist of linked polyhedra containing six- and four-, and less commonly five-coordinate molybdenum(VI). (Fig. 2)

Molybdenum trioxide and the molybdates of colourless cations are themselves colourless. Blue or green colours are due to the presence of reduced impurities except for those molybdates where the cation is coloured, e.g. violet cobalt(II) molybdate, CoMoO4, and green nickel(II) molybdate, NiMoO4.

Structures of Molybdates

Fig. 2: Structures of the heptamolybdate ion (top) and the octamolybdate ion (bottom) as linked atoms (Mo, filled circle; O, open circle) and linked MoO6 octahedra. Note that in the octamolybdate structure one octahedron is hidden.

In aqueous solution molybdate and polymolybdate ions are in rapid equilibria; the species depends on the pH and the concentration (see Table: Mo(VI) Species in Aqueous Solution) . The molybdate species in aqueous solutions of molybdenum(VI) (speciation) are listed in Table: Mo(VI) Species in Aqueous Solution.

Mo(VI) Species in Aqueous Solution
pHMo concentration/ mol l -1Mo(VI) species
>7 all [MoO4] 2-
5-6 >10 -3 mol l -1 [Mo7O24] 6-
3-5 >10 -3 mol l -1 [Mo8O26] 4-
0.9 >10 -3 mol l -1 MoO3 precipitates

Table: Mo(VI) Species in Aqueous Solution

Which compounds crystallise from solution depends on the conditions: concentration and pH. To prepare sodium molybdate, Na2MoO4.2H2O molybdenum trioxide is dissolved in sodium hydroxide solution at 50-70oC. The solution is filtered and the filtrate is crystallised in a batch crystalliser. The hydrated salt is dehydrated at 100oC. Ammonium dimolybdate, (NH4)2Mo2O7, is prepared by dissolving MoO3 in aqueous ammonia solution and crystallising the solution at ca 100oC by flash evaporation.

Structure of Ammonium Dimolybdate
Fig 3: Structure of the polymeric unit of ammonium dimolybdate

The structure of ammonium dimolybdate (Fig. 3) determined by X-ray crystallography on crystals from the manufacturing plant, consists of infinite chains of pairs of edge-shared MoO6 octahedra, adjacent pairs being linked by MoO4 tetrahedra with the chains having equal numbers of octahedra and tetrahedra. The structure is quite different from that of dichromate which contains discrete [Cr2O7] 2- ions. Ammonium heptamolybdate, (NH4)6Mo7O24.4H2O, is prepared by crystallising a solution of MoO3 in aqueous ammonia of the requisite NH3/Mo/H2O stoichiometry at ambient temperature. The structure comprises linked [MoO6] octahedra only.

If molybdates are to be used in organic media then they are in the form of suspensions although some solubility of molybdates in organic solvents (alcohols and ketones) can be achieved by replacing sodium and ammonium balancing cations with alkylammonium cations. The alkylammonium molybdates are salts - not complexes. The compound tetrakis(isopropylammonium) octamolybdate(VI), [C3H10N]4[Mo8O26] is typical of this type of compound.

The heteropolymolybdates consist of [MoO6] octahedra incorporating atoms of a different element, the heteroatom. The heteroatoms are completely surrounded by the oxygen atoms of the [MoO6] octahedra. The resulting coordination of the heteroatom may be tetrahedral or octahedral. The 12-molybdo species,
[X n+MO12O40] (8−n)-, is an important group with tetrahedrally coordinated heteroatoms (X). An example is 12-molybdophosphoric acid H3[PMo12O40] .28 H2O, prepared by dissolving molybdenum trioxide in phosphoric acid; it is yellow and readily soluble in water. Ammonium 12-molybdophosphate, which precipitates when ammonium molybdate is added to a solution of disodium hydrogen phosphate, is used for the gravimetric determination of phosphate.

A number of heteropolymolybdates are soluble in organo-oxygen solvents, e.g. 12-molybdosilicic acid and 12-molybdophosphoric acid.

Lower oxidation states

Molybdates(VI) unlike chromates(VI) are not strong oxidising agents: much of the familiar chemistry of molybdenum is that of the VI and V oxidation states. The products of the reduction of molybdate in aqueous solution depend on the pH and the reducing agent. In alkaline solution molybdates are reduced by, e.g. sodium dithionite, to molybdenum blues, mixed Mo(VI)-Mo(V) oxides. In dilute hydrochloric and sulfuric acids tin (II) chloride effects reduction to brown molybdenum(V) species; in concentrated hydrochloric acid the green [MoOCl5]2− species is formed. With a more powerful reducing agent, e.g. zinc amalgam, reduction proceeds to hydrated molybdenum(III) species.

Compounds of molybdenum(II) and lower oxidation states are stabilised by Mo-Mo bonds as in molybdenum(II) acetate, Mo2(CH3CO2) 4, and molybdenum(II) chloride, Mo6Cl12, and by unsaturated ligands, for example, molybdenum(0) hexacarbonyl, Mo(CO) 6. There is an extensive and important organometallic chemistry of molybdenum, i.e. compounds with Mo-C bonds with, e.g. cyclopentadiene and benzene.