Molybdenum Dichloride, MoCl₂

The normal chloride of divalent molybdenum, MoCl₂, which in solution would yield the simple ions Mo^•• and Cl', is not known. The so-called " molybdenum dichloride," which does contain the divalent metal, has been shown to be an amphoteric compound of more complex composition. It may be prepared by heating to redness the trichloride in a stream of dry carbon dioxide, when the volatile tetrachloride distils, leaving a residue of the dichloride. Careful regulation of the temperature, and exclusion of air and moisture, are important, or the product will contain higher chlorides and oxychlorides. The dichloride is also formed by heating molybdenum with mercurous chloride, or by passing chlorine largely diluted with carbon dioxide over the moderately heated metal; in both cases satisfactory yields are difficult to obtain. The most productive method appears to be the heating of molybdenum powder in a stream of carbonyl chloride at 610° to 620° C. The dichloride remains as a heavy yellow powder, which may be extracted with ether containing 5 per cent, of absolute alcohol. After evaporation of the solution in vacuo, a light yellow stable powder, of composition Mo₃Cl₆.C₂H₅OH, remains, the alcohol in which cannot be removed by heating in an inert atmosphere without further decomposition taking place. The addition of alcoholic silver nitrate to a solution of the compound in alcohol precipitates silver chloride, and the mother-liquor on standing deposits the compound Mo₃Cl₄(NO₃)₂.C₂H₅OH; immediate treatment of the mother-liquor with ether, however, causes separation of the alcohol-free compound Mo₃Cl₄(NO₃)₂. The formula for the chloride may therefore be written (Mo₃Cl₄)Cl₂.

If the original dichloride powder is extracted with concentrated hydrochloric acid, long yellow needles, of composition Mo₃Cl₆.HCl.4H₂O, may be separated almost quantitatively from the solution. These in air lose hydrogen chloride, becoming green; when heated in a stream of hydrogen chloride the compound Mo₃Cl₆.H₂O results. The yellow crystalline substance acts as a chloracid containing the anion Mo₃Cl₇', its solution in the minimum quantity of hydrochloric acid yielding, with concentrated solutions of metallic chlorides, salts of composition R^•[Mo₃Cl₇].xH₂O. The addition of dilute acid to a solution of the chloracid in dilute alkali precipitates a crystalline hydroxide of composition [Mo₃Cl₄.2H₂O](OH)₂.6H₂O. This formula is based on the behaviour of the compound on dehydration; at 100° C. 6 molecules of water are lost, a seventh is expelled below 300° C., and the final product, which resists further dehydration, has the composition [Mo₃Cl₄.2H₂O]O. With hydrobromic acid the hydroxide yields acids for which the following formulae have been given: H[Mo₃Cl₄Br₃.H₂O]. 3H₂O and H₂[Mo₃Cl₄Br₄.4H₂O]2H₂O; salts of these acids have also been described.

Molybdenum "dichloride," that is, the chloracid HMo₃Cl₇.4H₂O, dissolves in aqueous alkali hydroxides forming yellow solutions which, on boiling, deposit the black dihydroxide Mo(OH)₂. The alcoholic solution conducts electricity, the molecular conductivity increasing on dilution; hydrogen, the oxychloride Mo₃Cl₄(OH)₂, and sometimes molybdenum are liberated at the cathode, whilst acetaldehyde and ethyl chloride are formed at the anode.