carbene
/ CAR-been /
A carbene is one of the strangest reactive intermediates: a neutral carbon atom bonded to only two other groups, leaving it with two non-bonding electrons and no overall charge. Carbon usually wants four bonds, so a carbene, with only two, is profoundly unsatisfied, both electron-poor (it has an empty orbital) and electron-rich (it has a lone pair) at the same time. This dual nature makes it ferociously reactive.
The two leftover electrons can sit in two ways. In a singlet carbene the two electrons pair up in one orbital, leaving another orbital empty, and the carbon takes a bent shape; this is the common kind for ordinary organic carbenes. In a triplet carbene the two electrons stay unpaired in separate orbitals, behaving like a tiny double radical. The simplest carbene is methylene, :CH2. A famous reaction is when a carbene inserts itself into a carbon-carbon double bond, adding both of its bonds at once to make a three-membered ring (a cyclopropane).
Carbenes show up in cyclopropanation reactions, in the chemistry of diazo compounds, and as stabilized N-heterocyclic carbenes that have become important ligands and catalysts in modern synthesis. They illustrate beautifully that carbon does not always follow the neat four-bond, full-octet picture of introductory chemistry; reactive intermediates routinely break those comfortable rules for a fleeting moment.
Generating dichlorocarbene :CCl2 in the presence of an alkene lets it add across the double bond in one motion, stitching a chlorinated three-membered ring onto the molecule.
A carbene adds both of its bonds across an alkene at once to build a cyclopropane ring.
A carbene is not an ion; it is neutral but has only six electrons around carbon, so do not confuse it with a carbocation (positive, also six electrons) or a carbanion (negative, eight electrons).