Five‐Membered Metallacyclic Allenoids: Synthesis and Structure of Remarkably Stable Strongly Distorted Cyclic Allene Derivatives

Metallocene lends a hand: Treatment of [bis(alkynyl)HfCp2] complexes with HB(C6F5)2 results in 1,1-hydroboration and subsequent carbon–carbon coupling to yield the corresponding metallacyclopenta-1,2-diene derivatives (see structure of the trimethylsilyl derivative; Hf brown, B red, Si blue, F green, C black). X-ray diffraction and a quantum chemical analysis reveal an electronic structure between a bent cyclic allene and a coordinated butenyne.