Silabenzene is one of those chemical curiosities. With little use except to theoretical organic chemists, it’s a benzene molecule with one carbon atom replaced by a silicon “pendant” atom. A heterocyclic aromatic.
For a long time it was believed to be unstable and impossible to isolate, having only been observed in matrix isolation. In fact, chemists tried for years to succesfully synthesise the stuff, only finally succeeding in 2000. They couldn’t even make silabenzene itself (as in my litle diagram here), because of its high reactivity. Instead they needed to use a steric protective group — in other words, they attached a bulky molecule to the silicon to stop anything getting close enough to react with it! The first silicon heterocycle created was 2-silanaphthalene, synthesised three years earlier in 1997.
Now benzene, as you may know, is a very stable molecule, thanks to its big delocalised cloud of π-bonded electrons. That means that in chemical reactions, it’s a lot easier to attach something to a benzene ring than to break it open. Carbon-carbon bonds are tough.
Silicon, on the other hand, is rubbish at forming π-bonds. It’s especially rubbish at forming π-bonds with carbon atoms. Even though silabenzene is actually aromatic, the carbon tends to suck all the electrons away from the silicon, making a highly polar bond which is quite easy to split apart. As a result, Si-C double bonds are highly reactive.
I have to wonder though, if it were possible to create one, how stable a polycyclic molecule like pyrene might be with a silicon atom trapped in the centre of it…