Boron diborane

A second example is the simplest hydride of boron, diborane (62 ). This has the structure... 

Pure boron in the form of a thin film can also be obtained by heating diborane to 1000 K ... 

Boron forms a whole series of hydrides. The simplest of these is diborane, BjH. It may be prepared by the reduction of boron trichloride in ether by lithium aluminium hydride. This is a general method for the preparation of non-metallic hydrides. 

Aluminium tetrahydridoborate is a volatile liquid. It is the most volatile aluminium compound known. It is covalent and does not contain ions but has a hydrogen-bridge structure like that of diborane, i.e. each boron atom is attached to the aluminium by two hydrogen bridges ... 

Other boron hydrides are known, most of them having the general formula B H + 4- for example pentaborane, B5H9, decaborane, BjqHi4. Each can be made by heating diborane in suitable conditions for example at 420 K, decaborane is obtained. Boron hydrides have been tried as rocket fuels. 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Figure 2-60. Soine examples of electron-deficient bonds a) diborane featuring B-H-B bonds b) diborane in a tentative RAMSES representation c) the orbital in a B-B-B bond (which occurs in boron cage compounds),...

There is a pronounced tendency for boron to become bonded to the less substituted car bon of the double bond Thus the hydrogen atoms of diborane add to C 2 of 1 decene and boron to C 1 This is believed to be mainly a steric effect but the regioselectivity of addition does correspond to Markovmkov s rule m the sense that hydrogen is the neg atively polarized atom m a B—H bond and boron the positively polarized one... 

Boron trifluoride is used for the preparation of boranes (see Boron compounds). Diborane is obtained from reaction with alkafl metal hydrides organoboranes are obtained with a suitable Grignard reagent. 

Sodium borohydride reacts with boron halides to form diborane [19287-45-7] 2 6 more conveniendy handled as the monomer... 

It is often necessary to iatroduce dopant atoms iato the epitaxial (epi) layers. Typically, the dopant sources are hydrides (qv) of the impurity atoms. Common dopants are boron hydride, ie, diborane(6) [19287-45-7] 2 6 p-ty e dopiag, and arsiae [7784-42-17, AsH, and phosphoms hydrides for n-ty e dopiag (11). For example ... 

For adding dopiag impurities duriag vapor-phase growth, a gaseous or easily vaporizable Hquid compound is metered, added to the siUcon source gas stream, and reduced along with the siUcon compound. Typical examples are diborane, 2 phosphine, and boron tribromide, BBr. ... 

Another commercial process yields high purity boron of greater than 99%. In this process boron hydrides, such as diborane, are thermally decomposed (4). Because only boron and hydrogen are present in the starting material, contamination is minimal, and a very uniform, submicrometer powder is formed by the gas nucleation process. 

BCl, BBr, and BI undergo exchange reactions to yield mixed boron hahdes. Exchange reactions also occur with trialkyl, triaryl, trialkoxy, or triaryloxy boranes and with diborane. Anhydrous metal bromides and iodides can be prepared by the exchange reaction of the metal chloride or oxide and BBr or BI (21)-... 

Localized Bonds. Because boron hydrides have more valence orbitals than valence electrons, they have often been called electron-deficient molecules. This electron deficiency is partiy responsible for the great interest surrounding borane chemistry and molecular stmcture. The stmcture of even the simplest boron hydride, diborane(6) [19287-45-7] 2 6 sufficientiy challenging that it was debated for years before finally being resolved (57) in favor of the hydrogen bridged stmcture shown. 

Dlbor ine(6). This compound is manufactured by Gallery Chemical Co. ia Gallery, Peimsylvania. Laboratory-scale preparations are given ia equations 4—64 5 6, of which the last may be the most convenient method. Diborane is the most important starting material for all the other boron hydrides. 

Only H2, BeH2, and Be(BH 2 have higher heats of combustion. When diborane is pyroly2ed above 100°C ia a sealed tube, it is decomposed to higher boron hydrides and hydrogen gas ia a complex sequence of reactions. This reaction has been investigated ia considerable detail (65). 

Despite the fact that many boron hydride compounds possess unique chemical and physical properties, very few of these compounds have yet undergone significant commercial exploitation. This is largely owing to the extremely high cost of most boron hydride materials, which has discouraged development of all but the most exotic appHcations. Nevertheless, considerable commercial potential is foreseen for boron hydride materials if and when economical and rehable sources become available. Only the simplest of boron hydride compounds, most notably sodium tetrahydroborate, NajBHJ, diborane(6), B2H, and some of the borane adducts, eg, amine boranes, are now produced in significant commercial quantities. 

Diborane(6), B2H. This spontaneously flammable gas is consumed primarily by the electronics industry as a dopant in the production of siHcon wafers for use in semiconductors. It is also used to produce amine boranes and the higher boron hydrides. Gallery Chemical Co., a division of Mine Safety AppHances Co., and Voltaix, Inc., are the main U.S. producers of this substance. Several hundred thousand pounds were manufactured worldwide in 1990. 

Boron Hydrides" under "Boron Compounds" in ECT 1st ed., Vol. 2, pp. 593—600, by S. H. Bauer, Cornell University "Boron Hydrides and Related Compounds" under "Boron Compounds," Suppl. 1, pp. 103—130, by S. H. Bauer, Cornell University "Diborane and Higher Boron Hydrides" under "Boron Compounds," Suppl. 2, pp. 109—113, by W. J. Shepherd and E. B. Ayres, Gallery Chemical Company "Boron Hydrides" under "Boron Compounds" in ECT 2nd ed., Vol. 3, pp. 684—706, by G. W. Campbell, Jr., U.S. Borax Research Corporation "Boron Hydrides and their MetaHo Derivatives" under "Boron Compounds," in ECT 3rd ed., Vol. 4, pp. 135—183, by R. W. Rudolph, The University of Michigan. 

Boron Compounds (Boron Hydrides)" in ECT 1st ed., Vol. 2, pp. 593—600, "Boron Compounds (Boron Hydride and Related Compounds)" Suppl. 1, pp. 103—130, by S. H. Bauer, Cornell University "Boron Compounds (Diborane and Higher Boron Hydrides)" Suppl. 2, pp. 109—113, byj. W. Shepherd and... 

In this compound, synthesized in the low temperature reaction between diborane and excess ammonia, the cationic boron is coordinatively saturated in a tetrahedral environment. More recendy, cations having boron in tricoordinate or dicoordinate environments have been observed. These cationic species, called borenium and borinum ions, respectively, have been reviewed (19,20). 

Boron tribromide [10294-33A], BBr, is used in the manufacture of diborane and in the production of ultra high purity boron (see Boron, ELEMENTAL BoRON COMPOUNDS). Anhydrous aluminum bromide [7727-15-3], AIBr., is used as an acid catalyst in organic syntheses where it is more reactive and more soluble in organic solvents than AlCl. Tballium bromide [7789AOA], TlBr, is claimed as a component in radiographic image conversion panels (39). 

Thakar and Subba Rao showed that reductions with diborane give the same result regardless of whether diborane is generated externally or produced internally by the action of NaBH4 and boron trifluoride or aluminum chloride. They found that alicyclic or dialkyl ketones are not reduced beyond the alcohol stage even under drastic conditions however, diaryl ketones are hydrogenolyzed under normal conditions, while aryl ketones or a,i5-unsatu-rated ketones are hydrogenolyzed, in part, under drastic conditions. 

With this as introduction, let us now look at the individual steps in more detail for the case of hydroboration-oxidation of 1-decene. A boron hydride that is often used is dibomne (B2H6). Diborane adds to 1-decene to give tridecylborane according to the balanced equation ... 

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