Doubly oxidised carbene tests the limits of the octet rule

An unprecedented four-electron carbon intermediate has been isolated and characterised by researchers in the US. The team used a two-step approach to remove the non-bonding electrons from a stable carbene intermediate to create a crystalline doubly oxidised carbene.

Carbenes were first discovered at the beginning of the 20th century but their extreme reactivity made them difficult to handle and they were widely dismissed as a laboratory curiosity. These reactive species contain an unusual six-electron carbon atom, bonded to two substituents and with two non-bonding electrons on the carbon centre. Careful choice of the bonded groups can stabilise this electron-deficient carbon and over the last 30 years carbenes have become a powerful tool in synthesis.