Reactivity of permethylscandocene derivatives with acetylene. Structure of acetylenediylbis(permethylscandocene), (.eta.5-C5Me5)2ScC.tplbond.CSc(.eta.5-C5Me5)2

Excess acetylene reacts with Cp* 2 Sc-R (Cp*=η 5 -C 5 Me 5 ; R=H, alkyl, aryl, alkenyl, alkynyl, amide) below −78°C to yield R-H and Cp* 2 Sc-C≡CH; the latter then reacts with excess C 2 H 2 to form polyacetylene. Cp* 2 Sc-C≡CH cleanly decomposes to Cp* 2 Sc-C≡C-ScCp* 2 , most likely via σ bond metathesis involving the Sc-acetylide and terminal C-H bonds for two molecules of Cp* 2 Sc-C≡CH. The structure of this unusual acetylenediyl-bridged dimer has been determined by X-ray diffraction methods