Tantalum complexes reactions with alkynes

In 1982, Cotton et al prepared the first phosphinocarbene tantalum complex 99 from the reaction of Ta2Cl6(SMe2)3 with bis(diphenylphosphi-no)acetylene. The most remarkable feature of this reaction is the formal dimerization of the alkyne moiety.91... 

There is some material of relevance in a review on niobium and tantalum coordination chemistry. 1 The thermal reactions of the ions M" " (M = Ta, Nb, NbO, NbN) with arenes in the gas phase results in cleavage of the arenes to form metallacyclopentadiene ion complexes via retro-alkyne cyclotrimerization mechanisms. ... 

A mononuclear tantalum-benzyne complex (121) has been prepared by thermolysis of 120 [Eq. (20)].14 An X-ray crystal structure was reported for 121. Bond lengths for the benzyne unit are given in Table III. Complex 121 exhibits a rich insertion chemistry similar to that of Ti, Zr, and Ru benzyne complexes. Insertion reactions of 121 with ethylene, 2-butyne, acetonitrile, and carbon dioxide give 122, 123, 124, and 125, respectively (Scheme 15). Diphenylacetylene does not couple with 121, presumably because of steric constraints. Reagents with acidic protons such as methanol or terminal alkynes cleave the Ta—C bond to give butyl isocyanide and carbon monoxide, but... 

The interaction of alkyne complexes with protic acids has been the most thoroughly studied of electrophilic reactions. When these reactions proceed to organic products, alkenes are formed, often stereospecifically. Schwartz and co-workers have studied the reactions of Cp2MH(alkyne) complexes of the early transition metals, Ta (Labinger et al, 1974) and Nb (Labinger and Schwartz, 1975). These complexes gave cis alkenes exclusively in nearly quantitative yield when treated with strong acids, as exemplified by the formation of cis-2-octene from the tantalum-octyne complex. Since the... 

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