Carboranes are electron-delocalized (non-classically bonded) clusters composed of boron, carbon and hydrogen atoms. Like many of the related boron hydrides, these clusters are polyhedra or fragments of polyhedra. Carboranes are one class of heteroboranes. In terms of scope, carboranes can have as few as 5 and as many as 14 atoms in the cage framework. The majority have two cage carbon atoms. The corresponding C-alkyl and B-alkyl analogues are also known in a few cases. Structure and bonding Carboranes and boranes adopt 3-dimensional cage (cluster) geometries in sharp contrast to typical organic compounds. Cages are compatible with sigma—delocalized bonding, whereas hydrocarbons are typically chains or rings. Like for other electron-delocalized polyhedral clusters, the electronic structure of these cluster compounds can be described by the Wade–Mingos rules. Like the related boron hydrides, these clusters are polyhedra or fragments of polyhedra, and are similarly classified as closo-, nido-, arachno-, hypho-, etc., based on whether they represent a complete (closo-) polyhedron or a polyhedron that is missing one (nido-), two (arachno-), three (hypho-), or more vertices. Carboranes are a notable example of heteroboranes. The essence, these rules emphasize delocalized, multi-centered bonding for B-B, C-C, and B-C interactions. Structurally, they can be considered to be related to the icosahedral (Ih) B12H122– via formal replacement of two of its BH– fragments with CH. Isomers Geometrical isomers of carboranes can exist on the basis of the various locations of carbon within the cage. Isomers necessitate the use of the numerical prefixes in a compound's name. The closo dicarbadecaborane can exist in three isomers: 1,2-, 1,7-, and 1,12-C2B10H12. Preparation Carboranes have been prepared by many routes, the most common being addition of alkynyl reagents to boron hydride clusters to form dicarbon carboranes. For example, the high-temperature reaction of pentaborane(9) with acetylene affords several closo-carboranes as well as other products: nido-B5H9 + C2H2 closo-1,5-C2B3H5, closo-1,6-C2B4H6, 2,4-C2B5H7 When the reaction is conducted at lower temperatures, an open-cage carborane is obtained: nido-B5H9 + C2H2 nido-2,4-C2B4H8 Other procedures generate carboranes containing three or four cage carbon atoms () Monocarba derivatives Monocarboranes are clusters with BnC cages. The 12-vertex derivative is best studied, but several are known. Typically they are prepared by the addition of one-carbon reagents to boron hydride clusters. One-carbon reagents include cyanide, isocyanides, and formaldehyde. For example, monocarbadodecaborate ([CB11H12]−) is produced from decaborane and formaldehyde, followed by addition of borane dimethylsulfide. Monocarboranes are precursors to weakly coordinating anions. Dicarba