Niobium carbene complexes

Pugh et al. [486] published an equally facile synthesis for the transition metal pincer carbene complexes of titanium(IV), vanadium(ll), chromium(ll), manganese(II), niobium(III) and uranium(IV). It consists of treating the free carbene with the respective transition metal halide or the pincer imidazolium salt with the respective metal bis-trimethylsilylamide (see Figure 3.160). 

When the metal is niobium, the two metal frontier orbitals, dy and dz2, are closer in energy and match the corresponding orbitals of triplet free carbene. Figure 10-4b provides a picture of the interactions involved (only the important orbital interactions are shown). The elections in the M-C bond are much more equally distributed between the metal and Ccarbene (because the energy levels of the metal and carbon orbitals are comparable) than with an electrophilic carbene complex. In resonance terminology, this means that structure 10 becomes an important contributor to the overall structure of this particular Schrock carbene complex.11... 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

Carbonyl Olefination Using Zirconium, Tantalum, Niobium, Molybdenum, and Tungsten Carbene Complexes... 

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