Other Compounds of Molybdenum

The instability of the electronic structure caused by the unfilled inner orbit of a transition metal leads to a very variable valency. Molybdenum exhibits valencies of 2,3,4,5 or 6 in different compounds, and it is considered that it has zero valency in its hexacarbonyl Mo(CO)g. Because of this variability in its valency, many of its reaction products are mixtures of compounds in which it has different valencies. In solid form such products may be quite homogeneous in composition, and best represented by a nonstoichiometric molecular formula. The hypothetical simple compounds present in such products cannot be separated readily from one another because valency shifts occur in processing. 

Apart from the oxides and sulphides, other groups of simple compounds are the halides, chalcogenides and molybdates, and there are a number of well characterised organic compounds. Simple halides can be prepared by direct reaction between molybdenum powder and chlorine, fluorine or bromine, and the pentachloride MoClg has been used as a chlorination catalyst. 

The important chalcogenides are the disulphide, diselenide and ditelluride, and their use in lubrication is described later. Other selenides and tellurides have also been identified and studied to a limited extent, but as a class these materials tend not to be stoichiometric. One group of relatively simple compounds is that of the molybdates. These are salts of ammonia or metals with molybdic acid H2M0O4. 

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Other compounds of molybdenum are used as protective coatings in materials used at high temperatures solders as catalysts as additives to animal feeds and as pigments and dyes in glasses, ceramics, and enamels. 

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