Cluster compounds tungsten halides

The chemical reduction of the higher Mo and W halides provides good yields of the octahedral clusters, but the mechanism is obscure. By contrast, chemical oxidation of zero-valent Mo and W leads to the bromo and iodo cluster species in poor yields but provides considerable insight into the formation of these cluster compounds. The reaction of the hexacarbonyls Mo(CO)6 or W(CO)6 with I2 at moderately low temperatures produces a mixture of metal halide phases (25, 16). In the reaction W(CO)6 with I2, lower nuclearity clusters have been isolated as reaction intermediates that lead to W6 species. The tri-, tetra-, and pentanuclear tungsten iodide species are obtained from W(CO)6 and I2 by varying reaction times and temperatures, and the... 

Tungsten halides, halogeno and 0x0 complexes, as well as cluster compounds are obtained by reaction with halides. W can also form carbonyl, tertiary phosphine, cyano, and dinitrogen complexes. 

The kinetics and thermodynamics of the exchange of halide ions (X) in cluster compounds of the type [MgCyXe have been reported for M = Mo and W. Replacement of chloride ion (X ) by bromide ion involves aquation in the rate-determining step followed by rapid bromide-ion anation of the aqua-intermediate. The activation energies for the forward and reverse reactions are both reported to be ca. 18 kcal mol. The molybdenum clusters are about ten times as reactive as the analogous tungsten clusters. 

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