Metal-alkylidynes metallacycles

Metal carbenes are also initiators for the metathetical polymerization of alkynes by, e. g., catalysts such as M0CI5 and WClg. Reaction of the metal carbene with an alkyne gives a metallacyclobutene intermediate, followed by opening of this intermediate metallacycle into a new metal carbene that in its turn can interact with another alkyne molecule, and so on eq. (13). Metathesis of acetylenes proceeds through reactions between an alkylidyne complex and an alkyne via a metallacyclobutadiene intermediate (eq. (14)). 

Insertion into metal carbon bonds for alkylidene [reaction (j)] and alkylidyne [reaction (k)] complexes that yield metallacycles will also be considered. 

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