Tungsten complexes acetylene

The molybdenum and tungsten acetylene complexes [M(RCCR)3 (CO)] and [W(RCCR)3 (MeCN)] are very interesting. All diphenylacetylene molecules in... 

In attempting to extend this reaction to derivatives of chromium and molybdenum, completely different results were obtained under comparable reaction conditions. Reaction of 3-hexyne with (CH CN) Mo(CO) did not appear to give a compound corresponding to the tungsten acetylenic complex, Hexaethylbenzene isolated from the reaction mixture indicates that the molybdenum complex causes cyclotrimerization of the acetylenic molecule. 

When alkynes are treated with catalytic amounts of a carbene complex, polymerization instead of metathesis can occur (Figure 3.44) [565,595,597,752-754]. The use of carbene complexes to catalyze alkyne polymerization enables much better control of the reaction than with heterogeneous or multi-component catalysts. Pure acetylene oligomers (n = 3-9) with terminal fcrf-butyl groups have been prepared with the aid of a tungsten carbene complex [755]. 

Catalysis by alkali metal ions has recently been reported as an alternative route. In an argon matrix, acetylene forms a n complex with the metal. On irradiation, it isomerizes to the vinylidene form, M C=CH2. When complexed with metals, vinylidene is much more stable, in the same way that metal carbenoids are generally much more stable than carbenes, and rearrangement of a tungsten alkyne complex to a tungsten vinylidene complex has been reported. ... 

The tungsten carbene complex [(CO)5W=CHAr] (Ar = Ph, p-tol) reacts with 2-butyne to give stilbenes and [(CO)5W(MeC=CMe)]. Terminal alkynes, such as Bu C=CH, are however polymerized. Vinylidene complexes are proposed as intermediates since the acetylene-vinylidene rearrangement has ample precedence in stoichiometric reactions (Scheme 26). 

The polymerization kinetics of terminal alkynes have been determined. The reaction is initiated by tungsten carbene complexes (10) the chelating olefin is displaced by acetylene as the first step in the reaction, followed by formation of... 

R = Me or The acetylene complex is even formed by tungsten(V) d electron... 

We recently reported the preparation of a new acetylenic complex of tungsten (1), This study has now been extended to other alkynes and the other subgroup VI-B metals. 

B3 NOVEL ACETYLENE TUNGSTEN CARBONYL COMPLEXES, D.P. Tate, J.M. Augl, W.M. Ritchey, B.L. Ross and J.G. Grasselli... 

In accordance with this alkylidene—metallacyclobutane mechanism, metathesis of alkynes has been performed using tungsten metallacarbyne complexes [36]. The analogy between the reactions of olefins and acetylenic hydrocarbons has led to the assumption that metallacyclobuta-dienes could be intermediates for this reaction. 

I 7.4. Acetylene complexes are fairly common however, Nielson et al. [73] reported the structure of a tungsten complex, see below. Construct the important valence orbitals for this compound and indicate the orbital occupation. 

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