Carborane
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. 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-CBH. 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: When the reaction is conducted  at lower temperatures, an open-cage carborane is obtained: Other procedures generate carboranes containing three or four cage carbon atoms () Monocarba derivatives. "Mono"carboranes are clusters with BC 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 ([CBH]) is produced from decaborane and formaldehyde, followed by addition of borane dimethylsulfide. Monocarboranes are precursors to weakly coordinating anions. Dicarba clusters. Dicarbaboranes can be prepared from boron hydrides using alkynes as the source of  the two carbon centers. In addition to the "closo"-CBH series mentioned above, several open-cage dicarbon species are known including "nido"-CBH (isostructural and isoelectronic with BH) and "arachno"-CBH. Syntheses of icosahedral "closo"-dicarbadodecaborane derivatives (RCBH) employ alkynes as the RC source and decaborane (BH) to supply the B unit. Classification by cage size. The following classification is adapted from Grimes's book on carboranes. Small, open carboranes. This family of clusters includes the nido cages CBH, CBH, CBH, CBH, and CBH. Relatively little work has been devoted to these compounds. Pentaborane reacts with acetylene to give nido-1,2-CBH. Upon treatment with  sodium hydride, latter forms the salt Na[1,2-CBH]. Small, closed carboranes. This family of clusters includes the closo cages CBH, CBH, CBH, and CBH. This family of clusters are also lightly studied owing to synthetic difficulties. Also reflecting synthetic challenges, many of these compounds are best known as their alkyl derivatives. 1,5-CBH is the only known isomer of this five-vertex cage. Intermediate-sized carboranes. This family of clusters includes the closo cages CBH, CBH, CBH, and CBH. Isomerism is well established in this family: 1,2- and 1,6-CBH, and 2,3- and 2,4-CBH, as well as in open -cage carboranes such as 2,3- and 2,4-CBH and 1,2 and 1,3-CBH. In general, isomers having non-adjacent cage carbon atoms are more thermally stable than those with adjacent carbons, so that heating tends to induce mutual separation of the carbon atoms in the framework. Carboranes of intermediate nuclearity are most efficiently generated by degradations starting with carborane as summarized in these equations, starting from CBH: In contrast, smaller carboranes are usually prepared by building-up routes, e.g. from pentaborane + alkyne. Chromate oxidation of this 11-vertex cluster results in deboronation, giving CBH. From that species, other clusters  result by pyrolysis, sometimes in the presence of diborane: CBH, CBH, and CBH. Icosahedral carboranes. The icosahedral charge-neutral "closo"-carboranes, 1,2-, 1,7-, and 1,12-CBH (respectively "ortho"-, "meta"-, and "para"-carborane in the informal nomenclature) are particularly stable and are commercially available. The 1,2-isomer forms first upon the reaction of decarborane and acetylene. It converts quantitatively to the 1,7-isomer upon heating in an inert atmosphere. The formation of the 1,2- to 1,12-isomer requires 700 °C, proceeding in ca. 25% yield. [CBH] is also well established. Reactions. Base-induced degradation of carboranes give anionic nido derivatives, which can  be employed as ligands for transition metals, generating metallacarboranes, which are carboranes containing one or more transition metal or main group metal atoms in the cage framework. Most famous are the dicarbollide, complexes with the formula M[CBH]. Research. Dicarbollide complexes have been evaluated for many applications for many years, but commercial applications are rare. The bis(dicarbollide) [Co(CBH)] has been used as a precipitant for removal of Cs from radiowastes. The medical applications of carboranes have been explored. C-functionalized carboranes represent a source of boron for boron neutron capture therapy. The compound H(CHBCl) is a superacid, forming an isolable salt  with protonated benzene, CH. It protonates fullerene, C.