From a cyclotrisilane to a cyclotriplumbane: Low coordination and multiple bonding in group 14 chemistry

In less than three decades the chemistry of low-coordinated compounds of the heavier elements of group 14 has made unforeseen and unprecedented advances. In the meantime not only have homonuclear double bonds been realized between all elements of this group but also the first examples of compounds analogous to the alkynes have been isolated. Similarly, syntheses of molecules containing heteronuclear bonds have been achieved. Analogues of the allenes as well as the first example of a spiropentasiladiene are of particular interest. The present short review covers mainly work done in the author's laboratories together with other relevant reports from the literature. It begins with the synthesis of hexa-tert-butylcyclotrisilane, which is cleaved photochemically into a short-lived silylene and a marginally stable disilene; in subsequent reactions both of these species can undergo cycloadditions to furnish a broad palette of mostly novel ring systems. Furthermore, the first molecules with conjugated Si=Si, Ge=C, and Ge=Ge double bonds have been obtained and their reactivities examined. The isolation of the first diplumbenes and a cyclotriplumbane exhibiting an unusual bonding situation as the first homocyclic compound of lead constitute a provisional end of these developments.