Molybdenum hydroxychlorides

Chlormolybdic" Acids.—The compound MoOCl2(OH)2, molybdenum hydroxychloride or dichlormolybdic add, is readily obtained by passing hydrogen chloride over any heated oxygenated compound of molybdenum. Fine white unstable needles are obtained, the aqueous solution of which deposits on evaporation amorphous molybdic anhydride. The substance is also soluble in ether, with which it forms a crystalline compound. An alternative method of preparation of dichlormolybdic acid consists in cooling a solution of molybdic acid saturated with hydrochloric acid. ... 

Molybdates combine also with other acidic oxides, forming a series of complex molybdates, the most important of which are probably the phosphomolybdates (see p. 163). The formation of such compounds as diehlormolybdic acid or molybdenum hydroxychloride, MoOCl2(OH)2 (see p. 126), and the sulphate M0O3.SO3 (see p. 160), illustrate the behaviour of molybdenum trioxide as a basic oxide. 

The hydroxychlorides Mo3Cl4(OH)2.2H20 and Mo3Cl4(OH)2.8H20, or [Mo3Cl4.2H20](OH)2.6H20 (see p. 124), are obtained by precipitation with acetic acid, before or after saturation with carbon dioxide respectively, of a solution of molybdenum dichloride in potassium hydroxide. 

Actually, the drop of pH is related to more complex reactions and species. Thus, in more sophisticated models, several hydrolysis reactions and metal chloride formation are taken into account but the selection of species and reactions is somewhat different from model to model. Oldfield and Sutton [94] and Watson and Postlethwaite [2] considered only hydroxides as the product of cation hydrolysis. Sharland [96] introduced simple metallic chlorides. The most complete set of species and reactions has been used by Bernhardsson et al. [4], which made available the thermodynamic data of a large number of species, including several iron, nickel, chromium, and molybdenum polycations as well as metal chlorides and hydroxychlorides. Gartland [19] used a more limited set of species (Table 10.3) selected among the Bernhardsson data. According to their experimental results, Hebert and Alkire [95] included Al(OH) " as the hydrolysis product in their model of the crevice corrosion of aluminum alloys. 

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