Boranes

Boranes is the name given to the class of synthetic hydrides of boron with generic formula BxHy. In the past, borane molecules were often labeled 'electron-deficient' because of their multicenter bonding (in which a pair of bonding electrons links more than two atoms, as in 3-center-2-electron bonds); this was done in order to distinguish such molecules from hydrocarbons and other classically bonded compounds. However, this usage is incorrect, as most boranes and related clusters such as carboranes are actually electron-precise, not electron-deficient. For example, the extremely stable icosahedral B12H122- dianion, whose 26 cluster valence electrons exactly fill the 13 bonding molecular orbitals, is in no actual sense deficient in electrons; indeed it is thermodynamically far more stable than benzene.BoraneBH3Diborane(6)B2H6arachno-Tetraborane(10) B4H10Pentaborane(9)B5H9Decaborane(14)B10H14Dodecaborate(12) B12H122−B18H22iso-B18H22 Boranes is the name given to the class of synthetic hydrides of boron with generic formula BxHy. In the past, borane molecules were often labeled 'electron-deficient' because of their multicenter bonding (in which a pair of bonding electrons links more than two atoms, as in 3-center-2-electron bonds); this was done in order to distinguish such molecules from hydrocarbons and other classically bonded compounds. However, this usage is incorrect, as most boranes and related clusters such as carboranes are actually electron-precise, not electron-deficient. For example, the extremely stable icosahedral B12H122- dianion, whose 26 cluster valence electrons exactly fill the 13 bonding molecular orbitals, is in no actual sense deficient in electrons; indeed it is thermodynamically far more stable than benzene. While some boranes are highly reactive with respect to electron-pair donors, others are not, e.g., the BnHn2- dianions (n = 6-12) as well as many neutral boranes such as B18H22. Some lower boranes are pyrophoric in air and react with water. The boranes belong to the class of cluster compounds, which have been the subject of developments in chemical bonding theory. A large number of the related anionic hydridoborates have also been synthesized. The development of the chemistry of boranes presented a number of challenges. First, new laboratory techniques had to be developed to handle these often pyrophoric compounds. Alfred Stock created the glass vacuum line, now known as a Schlenk line, for synthesis and handling. The very reactive nature of the lower boranes meant that crystal structure determination was impossible before William Lipscomb developed the requisite techniques. Lastly, once the structures were known it became clear that new theories of chemical bonding were needed to explain them. Lipscomb was awarded the Nobel prize in Chemistry in 1976 for his achievements in this field. The correct structure of diborane was predicted by H. Christopher Longuet-Higgins 5 years before its determination. Polyhedral skeletal electron pair theory (Wade's rules) can be used to predict the structures of boranes. Interest in boranes increased during World War II due to the potential of uranium borohydride for enrichment of the uranium isotopes. In the US, a team led by Schlesinger developed the basic chemistry of the boron hydrides and the related aluminium hydrides. Although uranium borohydride was not utilized for isotopic separations, Schlesinger's work laid the foundation for a host of boron hydride reagents for organic synthesis, most of which were developed by his student Herbert C. Brown. Borane-based reagents are now widely used in organic synthesis. Brown was awarded the Nobel prize in Chemistry in 1979 for this work. Borane clusters are classified as follows, where n is the number of boron atoms in a single cluster: The International Union of Pure and Applied Chemistry rules for systematic naming is based on a prefix denoting a class of compound, followed by the number of boron atoms and finally the number of hydrogen atoms in parentheses. Various details can be omitted if there is no ambiguity about the meaning, for example, if only one structural type is possible. Some examples of the structures are shown below.