A linear C=Ge=C heteroallene with a di-coordinated germanium atom

Allenes (>C=C=C<) are classified as cumulated dienes that have a linear structure with an sp-hybridized central carbon atom. We have succeeded in synthesizing and isolating a stable 2-germapropadiene derivative with bulky silyl substituents. The 2-germapropadiene allene moiety adopts a linear structure both in the solid state and in solution. In addition, an X-ray diffraction electron-density-distribution analysis of this 2-germapropadiene revealed a linear C=Ge=C geometry with a formally sp-hybridized germanium atom bearing two orthogonal C=Ge pi-bonds. Based on detailed structural and computational studies, it can be concluded that the linear geometry of the isolated 2-germapropadiene most likely arises from the negative hyperconjugation of the silyl substituents at the terminal carbon atoms. The 2-germapropadiene reacts rapidly with nucleophiles, indicating that the linearly oriented germanium atom is highly electrophilic.