Atomistry » Molybdenum » Compounds
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Molybdenum Compounds

In its most stable and most important compounds, molybdenum occurs as a hexavalent element. The trioxide MoO3, corresponding to chromium trioxide, is markedly acidic, forming salts which readily combine with further molecules of the acid oxide to form series of poly molybdates. The strongly electronegative character of hexavalent molybdenum is also exhibited in the acid nature of the sulphide, and the formation of thiomolybdates of the types R2S.MoS3 and R2S.2MoS3, and again in the difficulties encountered in attempts to isolate halogen compounds of the type MoCl6, owing to the ready formation of complex anions containing molybdenum and the halogen. The trioxide readily combines with other acidic oxides, such as phosphorus pentoxide, arsenic pentoxide, and silica, forming series of complex heteropolyacids which give rise to well-defined crystalline salts. These compounds, in colour, isomorphism, and general properties, show a close relationship to the corresponding derivatives of tungsten.

The compounds containing molybdenum in lower stages of valency are much less stable than the fully oxidised derivatives, and in many cases further investigation is necessary before their constitution can be expressed with certainty. Normal salts containing divalent molybdenum are not known, but certain halogen compounds of amphoteric character do exist, of empirical formula MoX2 (X = Cl, Br, I), but of molecular formula Mo3X6. These are insoluble in water, but the chloride and bromide yield acids of the type HMo3X7, salts of which have been prepared. Substituted compounds, such as Mo3Cl4(OH)2 and Mo3Cl4(NO3)2, are also known. None of these substances in solution yield simple halogen ions, and the stability of the complex is indicated by the fact that they are unaffected by such oxidising agents as potassium nitrate.

Trivalent molybdenum is found in a few simple compounds. The black hydroxide, Mo(OH)3, dissolves in acids with salt formation, yielding reddish-purple solutions which darken in colour. Upon evaporation crystalline salts are not obtained, but when the solution is taken to dryness a greyish-black residue remains, which can be re- dissolved in water to a dark grey solution. This may be accounted for by the readiness with which the salts undergo hydrolysis, with formation of the black hydroxide, possibly in the colloidal form. The similarity of molybdenum to chromium is seen in the series of complex thiocyanates of the type R3[Mo(CNS)6].aq., which are analogous to, and isomorphous with, the chromithiocyanates R3[Cr(CNS)6].aq.

Few derivatives of molybdenum dioxide, MoO2, have been prepared, and it is doubtful whether simple salts containing tetravalent molybdenum can be formed in solution. By the electrolytic reduction of acid molybdate solutions, brownish-coloured liquids apparently containing the metal in this stage of oxidation have been obtained, but the evidence is insufficient to determine whether MoIV ions are actually present, or whether the liquids merely contain mixed MoV and MoIII ions. Potential measurements indicate the presence of mixed ions. The only simple substances containing tetravalent molybdenum, in addition to the oxide, are the sulphide, MoS2, the tetrachloride, MoCl4, and the tetrabromide, MoBr4. There are, however, two series of complex molybdo-cyanides, of the types R4[Mo(OH)4(CN)4].aq. and R4[Mo(CN)8].aq. respectively, which contain tetravalent molybdenum and yield well-crystallised salts. Their existence is probably due to the resistance of the stable complex to hydrolytic decomposition.

Pentavalent molybdenum occurs in the hydroxide Mo(OH)5, which is obtained as a reddish-brown precipitate by addition of alkali to the red solution resulting from careful reduction of aqueous molybdic acid. The pentahydroxide is basic, but the only normal salt as yet obtained is the pentachloride, which is prepared in the dry way. Salts containing the molybdenyl radical MoO••• are obtainable in the wet way; for example, a series of halogen double salts, of the type (X = F, Cl, Br), is known. Complex thiocyanates, of the form Mo(OH)2(SCN)3, have been obtained in combination with organic bases, whilst molybdicyanides, of the type R3Mo(CN)8, analogous to the corresponding tungsten compounds, have been described.

Highly oxidised compounds of molybdenum, usually obtained through the agency of hydrogen peroxide, are known; such, for example, are the permolybdates, fluoxypermolybdates, and the perthiomolybdates. In these compounds, however, the valency of the molybdenum appears to be 6, which may be considered as the maximum valency of the element. The constitution of the sulphur compounds, which are of the types RMoS5 and RMoS6, has not as yet been established, and although a tetrasulphide, MoS4, exists, there is no evidence that the molybdenum is functioning with higher valency than 6.

Molybdenum and Oxygen

Oxides of molybdenum corresponding to the formulae Mo2O3, MoO2, and MoO3 are known to exist; that represented by the last formula, an acid-forming oxide, has been studied in greatest detail. The intermediate blue oxide of molybdenum, obtained by the reduction of the trioxide, is well known, but its composition cannot be considered to be satisfactorily settled; the formula Mo3O8 is usually ascribed to it.

Molybdates of the Rare Earth Metals

Salts of the type M2(MoO4)3 have been described. The cerous salt is obtained as yellow crystals by fusing together anhydrous cerous chloride and sodium molybdate. The density of the molten salt is 4.56. The crystals are similar to those of lead and bismuth molybdates, as also are those of "didymium" molybdate.

A number of complex molybdates have been prepared containing the rare earth elements of the cerium group. They have the general formula (NH4)6M•••2Mo14O48.24H2O, and form a series of isomorphous salts crystallising in the triclinic system. They may be considered as double salts of the paramolybdate series. A well-defined series of complex cerimolybdates, analogous to the complex salts of thorium and zirconium, and derived from an acid of the type, has been prepared. The salts do not give the reactions for Ce••••; for example, they are unattacked by hydrogen peroxide, so that the cerium appears to be present in a complex ion, and the formula H8[Ce(Mo2O7)6]aq. has been suggested. The neutral ammonium salt, (NH4)8[Ce(Mo2O7)6].8H2O, is obtained as yellow crystals by treating an aqueous solution of ammonium paramolybdate with a solution of cerium ammonium nitrate. By the action of dilute sulphuric acid on a solution of the normal salt, the acid salt, (NH4)6H2[Ce(Mo2O7)6].10H2O, has been prepared, and by double decomposition the silver salt, Ag8[Ce(Mo2O7)6], may be isolated.

Molybdenum and Nitrogen

Molybdenum chloride and dry ammonia react at a fairly low temperature with formation of ammonium chloride and a grey substance which, after extraction with water to remove ammonium and molybdenum chlorides, and warming in air, is spontaneously inflammable. The composition, however, of such a substance, or of that obtained by sublimation of molybdenum chloride with ammonium chloride, is apparently not definitely known.

It has been found that when molybdenum oxide, hydroxide, or a mixture of either of these compounds with the metal, is heated at 500° to 600° C. with equal parts of nitrogen and hydrogen under a pressure of 60 atmospheres, there is formed a nitride of molybdenum. The same compound may be obtained under atmospheric pressure by reducing pure precipitated molybdic acid by means of hydrogen at 700° C., and then passing nitrogen over the product at the same temperature. The commercial importance of this substance lies in the fact that on heating in a vacuum it yields pure metallic molybdenum, with simultaneous liberation of ammonia.

The existence of a compound, MoO2(NH2)2, molybdamide, obtained by interaction of ammonia with the oxychloride MoO2Cl2, and yielding ammonia when treated with water, has been noted.

Nitrates of molybdenum are not known. By digesting dilute nitric acid with excess of molybdenum, or with the hydrate of molybdenum dioxide or sesquioxide, a dark brown solution is obtained, which on further evaporation yields oxides of nitrogen and molybdic acid.

Molybdenum and Arsenic

Molybdenum arsenide has not been prepared. Molybdous arsenate is formed as a grey precipitate when molybdous chloride is treated with sodium arsenate; the precipitate first redissolves but afterwards becomes permanent. Molybdic arsenate, obtained in a similar manner from molybdic chloride, has been described by Berzelius, who also considered that an acid salt was produced on dissolving molybdic hydrate in excess of arsenic acid, since the solution turned blue on standing.

Molybdic acid forms with arsenic acid a series of heteropoly-acids analogous to the molybdophosphoric acids described above. When a mixture of molybdic acid, arsenic acid, and an ammonium salt is boiled for some time, a yellow crystalline precipitate is obtained of an ammonium molybdo-arsenate. To this compound Debray gave the formula 3(NH4)2O.As2O5.20MoO3, and considered that on boiling with aqua regia and evaporating the solution, the residue contained two acids - one yellow, in which the ratio of As2O5 to MoO3 was 1:20, and the other white, with As2O5:MoO3 as 1:16. This was contested by Seyberth, who considered that both the precipitate and the acid obtained from it contained As2O5:MoO3 = l:7. Various other acids and salts have been described, in which the ratio of As2O5 to MoO3 differs considerably, those compounds rich in molybdic acid generally being yellow in colour, while those containing less molybdic acid are white. A satisfactory formulation of these compounds, showing their analogy to the molybdophosphates, is largely due to Rosenheim and his co-workers.

Molybdenum and Cyanogen

The simple cyanides of molybdenum do not appear to have been isolated, but a large number of complex compounds have been described. It was observed by Pechard that molybdenum dioxide dissolves in aqueous potassium cyanide forming a strongly alkaline blue liquid, which on concentration deposits blue needles, to which he gave the formula MoO2(CN)2.2KCN. The compound is readily soluble in water, and gives with solutions of metallic salts characteristically coloured precipitates: bluish white with lead, pale brown with copper, greenish blue with mercuric salts, and dark brown with silver salts. Several complex compounds, apparently containing tetravalent molybdenum, may be obtained by reduction of potassium molybdate in presence of potassium cyanide. For example, by reduction with hydroxylamine hydrochloride, violet crystals, of composition MoO2.4KCN.NH2OH + H2O, are obtained, while on neutralising a solution of the oxychloride, MoOCl3, with potassium hydroxide and adding excess of potassium cyanide, a blue solution results which, on further addition of potassium hydroxide, yields a bluish-red crystalline precipitate, of composition MoO2.4KCN.10H2O. This compound may also be obtained by the addition of a large excess of potassium cyanide to an aqueous solution of the compound Mo(OH)2(SCN)3.2C5H5N; when only a slight excess of potassium cyanide was used, the compound K4Mo(CN)8.2H2O was obtained in yellow plates. The latter compound was first obtained by the action of potassium cyanide on the double chloride of potassium and molybdenum; it is readily soluble in water, and is stable towards acids and dilute alkalies. When the aqueous solution is exposed to direct sunlight, its colour changes to red and then to pale green, hydrogen cyanide being formed.

The composition of this compound is particularly interesting, since it was recognised as the first compound with a stable complex ion of which the co-ordination number was greater than 6. The valency of the molybdenum present was for some time in doubt, for whilst the empirical formula suggested that the metal should be tetravalent, titration with permanganate indicated that it was in the pentavalent condition; it was shown by Olsson, however, that permanganate oxidised the compound, not to molybdic acid, but to a complex cyanide of the type R3Mo(CN)8, in which the molybdenum is pentavalent. Olsson was unable to isolate this compound, but he arrived at his conclusions through the analogous tungsten compounds, and he did succeed in isolating the salts of this metal of the type R3W(CN)8.xH2O.

Two definite series of complex molybdocyanides, containing tetravalent molybdenum, are now recognisable: (1) the reddish-violet salts, containing the complex anion

which on evaporation with water are converted to blue salts containing the anion


and (2) the yellow salts, obtained by the action of alkali cyanides on the blue salts, of the type R4[Mo(CN)8].

Molybdenum and Boron

When molybdenum and boron are heated together in the electric furnace crystalline borides are produced. By heating a mixture containing 6 grams of metallic molybdenum, obtained by reduction of the trioxide and 1 gram of boron, Tucker and Moody obtained a homogeneous product which was free from carbon and silicon. It was crystalline in structure, of hardness 9 and of density 7.105. It was quite brittle, and on fracture showed a brilliant metallic lustre similar to that of pale brass. Hot concentrated acids attacked it slowly, but hot aqua regia acted more vigorously. Analysis showed the presence of 86 per cent, of molybdenum, and the formula Mo3B4, which requires 86.7 per cent. Mo, was suggested. These experiments, however, were repeated by Wedekind, who obtained a product containing 88 per cent, of molybdenum and 9.9 per cent, of boron, so that the molybdenum content was too high to agree with the above formula, especially in view of the fact that the boride contained impurities. Wedekind also produced an impure boride, having the approximate composition Mo2B, by passing an arc between electrodes (made by submitting mixtures of finely powdered molybdenum and boron to hydraulic pressure) in a vacuum electrical furnace; the heat brought about the formation of the boride with disintegration of the electrodes.

Non-crystalline alloys of molybdenum and boron have been obtained by heating together molybdenum dioxide and boron in magnesia crucibles. These alloys, containing up to 46 per cent, of boron, decrease in density and increase in hardness with increase in the percentage of boron. They are not attacked by hydrochloric and hydrofluoric acids or by alkalies, but concentrated sulphuric acid acts on warming, and dilute nitric acid dissolves them in the cold.
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