Bonding and reactivity of heavier group 14 element alkyne analogues

In this short review recent developments in the synthesis and spectroscopic characterization, as well as the bonding and reactivity, of heavier group 14 element alkyne analogues (REER, R = large aryl or silyl group; E = Si-Pb) are summarized. Stable(1) examples of such compounds have been known only since 2000, although their bonding has been studied by theoretical methods since the early 1980s. The use of very large substituents has enabled their isolation and the study of their reactions at room temperature. All the currently known compounds possess trans-bent planar core structures and nonbonded electron density at the tetrel (group 14 element). The bending increases with increasing atomic number, and when E = Pb the triple bonding seen in the lighter carbon homologue has been transformed into a single bond and two nonbonded electron pairs. To a crude approximation the bond order decreases by ca. 0.5 of a unit between elements upon descending the group. Currently available reactivity data show that the silicon and germanium derivatives are significantly more reactive than their tin or lead analogues. A digermyne has been shown to react directly with hydrogen under ambient conditions, suggesting the presence of some singlet diradical character. The distannynes have been shown to adopt either a multiply bonded structure similar to its germanium analogue or a singly bonded structure like the lead species. Much further work will be required to obtain a clearer picture of this exciting new class of molecules.