Credit: © 2008 Wiley

Silenes are compounds that contain a carbon atom double-bonded to a silicon atom and are quite rare — indeed, the first stable one was only synthesized in 1982. Silicon, like carbon, has four valence electrons with which to bond, but the difference in electronegativity between the two means C=C double bonds are much more common than Si=C. And whereas metal-substituted alkenes are common reagents, no metal-substituted silenes are known.

Now, Yitzhak Apeloig, Dmitry Bravo-Zhivotovskii and colleagues from the Israel Institute of Technology in Haifa (Technion) have prepared and characterized1 the first metal-substituted silene. The compound, made using a thermal rearrangement reaction, is also the first example of a bis-silene — a molecule with two Si=C bonds — to be isolated. The crystal structure of the bis-silene reveals that the two Si atoms that are bonded to the central Hg atom are planar, implying that they are sp2 hybridized and, therefore, silenes rather than silanes. Although the compound is stable towards water, methanol and acetone for several weeks, the bis-silene reacts instantaneously when exposed to air. It is thought that the stability is a consequence of the large isopropyl and adamantyl groups surrounding the silicon–metal centre.

Further investigations revealed that the size of the substituent groups has an important role in the synthesis in that high levels of steric crowding are necessary to even make the bis-silene — the final rearrangement step becomes more endothermic as the size of the substituents decrease. Because silyl mercury compounds are photosensitive, Apeloig and co-workers are exploring the photochemistry of the bis-silene in the hope that it leads to more new silicon compounds.