Trivalent and Divalent Compounds

Of the rather limited number of tetravalent molybdenum and tungsten derivatives known, wre shall mention here only molybdenum disulfide (M0S2), which occurs as the ore molybdenite and the octacyano complexes Mo(CN)i 4 and W(CN) 4 which, along with their pentavalent counterparts (p. 333), represent two of the very few examples of octa-covalency. 

A small number of compounds of Mo(III), W(III), and U(III) are known, and they are very easily oxidized to higher valence states. Tri-valent molybdenum and tungsten are reducing agents comparable with 

The nature of metal carbonyls has already been indicated (p. 159). The hexacarbonyls formed by the Group Via elements are white solids whose boiling points lie between 145° C (for Cr(CO)6) and 175° C (for W(CO)e) each of the molecules is octahedral. Since each of the metals precedes a rare gas in the Periodic Table by twelve spaces, these carbonyls are in accord with the Sidgwick-Bailey rule (p. 158). The failure of all attempts to prepare uranium hexacarbonyl, U(CO)e, was taken as an early indication that the period following radon was not an 18-membered period (a good example of reaching a correct conclusion through a somewhat questionable assumption). 

The method of choice for preparation of these carbonyls is the reaction between carbon monoxide, phenylmagnesium bromide (CeH6MgBr), and an appropriate anhydrous chloride (OCI3, M0CI3, or WCle). The reaction involving CrCU has been investigated in some detail in the absence of carbon monoxide, several possible intermediates can, with 

Sidgwick, N. V. The Chemical Elements and Their Compounds, 998-1086, Oxford University Press, London (1950). 

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