Tungsten divalent

In keeping witii its 5d 6s2 electron configuration, tungsten forms many compounds in which its oxidation state is 6+, just as molybdenum does. It forms divalent and tetravalent compounds to about the same extent as molybdenum but its bivalent and pentavalent compounds are somewhat fewer. Its anion chemistry is closely akin to that of molybdenum. 

The divalent ions Mg2+, Ca2+, and Zn2+ have long been known to serve as Lewis acids in important biological processes [223,224], As Lewis acids, these metals either activate substrate toward hydrolysis or activate a nucleophile such as water or serine. The high-valent tungsten site in acetylene hydratase may play one (or both) of these roles. 

Owing to the slow rates of diffusion of the cations, the direct solid-state reaction of the oxides Cr2 03 and MO at an elevated temperature is not a good preparation of divalent metal chromium(III) oxides. They can be prepared by more elaborate methods, such as controlled reduction of dichromates MCr207,1 reaction of dichromium tungsten oxide Cr2W06 with a molten divalent metal fluoride2 at 1400°, pyrolysis of complexes,3 and pulverization of slurries containing Cr2 03 and a divalent metal salt.4... 

In the fused state, the straightforward method is to allow either the lanthanide metal or the alkaline earth metal to react with a melt of the alkaline earth halide and the lanthanide trihalide. This approach yields the desired divalent ions if inert containers are used. Satisfactory reduction has been obtained in molybdenum, tungsten, and tantalum. The... 

In divalent tungsten complexes besides monomer configiuations, also dimeric configurations having a quadruple W-W bond are known. 

While the alkaline digestion of wolframite ores results in the formation of Mn(OH)2 (equation 1) in which Mn is divalent, the presence of Mn02 in the ore tailings is not surprising because the air oxidation of Mn(OH)2 is well known (5). Extensive observations of the dissolution behavior of the tungsten ore tailings indicate that it is not a simple acid dissolution but a redox reaction such as... 

Gloss and Moss have suggested the term carhenoid to describe intermediates that exhibit reactions quahtatively similar to those of carbenes without necessarily being free divalent carbon species. When carbenes are coordinated with metals they are known as carbenoids that have modified reactivities. lodomethylzinc iodide is often referred to as a carhenoid, because it resembles a carbene in its chemical reactions (Simmons-Smith reaction) (Scheme 5.14). Rhodium and copper carbenoids are unstable whereas some transition metals such as tungsten and chromium form stable and isolable carbenoids called metallocarhenes or Fischer carbenes. Carbenoids are structurally related to singlet carbenes and possess similar reactivity. 

Different Reactivities of Divalent Silicon Compounds Towards Metallocene Derivatives of Molybdenum and Tungsten... 

Keywords Divalent Silicon Compounds / Molybdenum Compounds / Silylene Complex / Tungsten Compounds / X-Ray Crystal Structures... 

---