In organic chemistry, propellane is any member of a class of polycyclic hydrocarbons, whose carbon skeleton consists of three rings of carbon atoms sharing a common carbon–carbon covalent bond. The concept was introduced in 1966 by D. Ginsburg Propellanes with small cycles are highly strained and unstable, and are easily turned into polymers with interesting structures, such as staffanes. Partly for these reasons, they have been the object of much research. Nomenclature The name derives from a supposed resemblance of the molecule to a propeller: namely, the rings would be the propeller's blades, and the shared C–C bond would be its axis. The bond shared by the three cycles is usually called the "bridge"; the shared carbon atoms are then the "bridgeheads". The IUPAC nomenclature of the homologue series of all-carbon propellanes would be called tricyclo[x.y.z.01,(x+2)]alkane. More common in literature is the notation means the member of the family whose rings have x, y, and z carbons, not counting the two bridgeheads; or x + 2, y + 2, and z + 2 carbons, counting them. The chemical formula is therefore . The minimum value for x, y, and z is 1, meaning three fused cyclopropyl-rings forming the [1.1.1]propellane. There is no structural ordering between the rings; for example, [1.3.2]propellane is the same substance as [3.2.1]propellane. Therefore, it is customary to sort the indices in decreasing order, . Further, heterosubstituted propellanes or structurally embedded propellane moieties exist and have been synthesised and follow a more complex nomenclature (see below). General properties Strain Propellanes with small cycles, such as [1.1.1]propellane or [2.2.2]propellane, bear a high absolute strain energy. The two interbridgeheaded carbons and their bonds may even be described as an inverted tetrahedral geometry. The resulting steric strain causes such compounds to be unstable and highly reactive. The interbridgehead C-C bond is easily broken (even spontaneously) to yield less-strained bicyclic or even monocyclic hydrocarbons. This so-called strain-release chemistry is used in strategies to access otherwise hard-to-obtain structures. Surprisingly, the most strained member [1.1.1] is far more stable than the other small ring members ([2.1.1], [2.2.1], [2.2.2], [3.2.1], [3.1.1], and [4.1.1]), which can be explained by special bonding situation of the interbridgehead bond. Bonding properties The bonding situation of small-ring propellanes, such as [n.1.1]propellanes, is topic of debate. Recent computational studies explain the interbridgehead bond as a Charge-shift bond possessing an unusual positive Laplace operator of the electron density . Studies by