Tungsten polynuclear complexes

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

The chemistry of tungsten is varied and complex not only because it covers nine oxidation states (-2 to +6), but also because of its ability to form complexes with different coordination numbers and geometries, and because of its tendency to form clusters and polynuclear complexes with a variety of metal atoms. 

Increasing the number of electrons reduces the activation of N2, because the electrons occupy the orbitals which are bonding with respect to the NN bond, and actually stabilize it. In agreement with this prediction dinitrogen is sufficiently activated to be reduced by protonation by dinuclear complexes of titanium(II), zirco-nium(Il), niobium(III), tantalum(III), molybdenum(IV), and tungsten(IV), whereas it is not reduced by protonation by certain d -d complexes, such as those of molybdenum(O), ruthenium(II), or rhodium(I). Apparently dinuclear complexes M-N=N-M in which M has the d electronic configuration can be intermediates in dinitrogen reduction in protic media, particularly if they represent part of polynuclear complexes (vide infra). 

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