The Subhalides of Boron

Lack of space prevents descriptions of a number of additional types of boron compounds such as the numerous interstitial metallic borides, the subhalides (of type B2CI4), the organoboron compounds (such as B(CH3)3), and the boron-nitrogen addition compounds, many of which have been prepared by H. C. Brown to study the effects of structures on the stability of Lewis acid-base adducts. ... 

Diboron tetrachlorides undergo a variety of reactions, including facile additions to aUcenes similar to hydroboration (equation 20). The reactions of boron subhalides, particularly B2CI4, with organic and organometallic compounds, have been studied in some detail. 

Boron subhalides are binary compounds of boron and the halogens, where the atomic ratio of halogen to boron is less than 3. The boron monohalides, BC1, [20583-55-5], bromoborane(l) [19961-29-6], BBr, and iodoborane(l) [13842-56-3], BI, are unstable species that have been observed spectroscopically when the respective trihalides were subjected to a discharge (5). Boron dihalide radicals have been studied, and structural and thermochemical data for these species ( BX2) have been deduced (5). 

Rednction of boron trihaUdes to elemental boron can be accomplished by heating with alkali metals, alkaline earth metals, or hydrogen. Under the proper conditions, rednctions of this type can also yield diborane and, under selected conditions, boron subhalides (see below). Metal hydrides also react with boron trihalides to give diborane. Boron nitride and boron carbide have been prepared by the high-temperature reductions of boron trihalides with ammonia and methane, respectively, and deposited on metal substrates by CVD. 

Group 13 clusters with electron counts less than the 2n - - 2 electrons corresponding to closo deltahedra have also been described. The classic examples of these are the boron subhalides, B C1 where n = 4, 8 -12 (and a more limited set of bromides and iodides), which have an electron count of 2n and are known as hypercloso clusters. These are based on the same closo structures expected for the 2n - - 2 electron clusters, namely, tetrahedral, dodecahedral, and tricapped trigonal prismatic for n = 4, 8, and 9. The 2n - - 2 dianions B C1 also show the expected deltahedral structures. The stability of the B C1 clusters has been attributed to a combination of the r-donor properties of the halides and steric effects. ... 

Studies of the general chemistry of compounds containing the B—B linkage have been reviewed (IJ, 56). The boron subhalides have also been discussed at some length in general reviews of boron halide chemistry (67, 101). 

Rather different, and higher, values of the boron-boron bond energies were obtained by Dibeler and his co-workers from photoionization mass spectrometric studies of B2F4 and B2CI4 and the corresponding trihalides. Essentially, this work combines known heats of formation of the subhalides (106) with threshold energies for dissociative ionization processes such as... 

Boron Halides.— Timms has published a short review on the preparation and properties of boron subhalides. ... 

Boron has a great affinity for oxygen and occurs in nature only in boric acid or borates. Borates are composed from clusters of flat trigonal BO3 and tetrahedral BO4 groups. The structural chemistry of borates is as rich and complicated as those of silicates, borides, or boranes. Boron oxide is an essential part of borosilicate glasses such as Pyrex. Boron halides are volatile molecular compounds. They are Lewis acids and react violently with water. The subhalides consist of boron chains or clusters that have terminally bound halogen atoms. They are substitution derivatives of the lower boranes. 

Similar reactions occur between boron subhalides and a number of chloro- and bromoolefins. The halogen originally present in the olefin appears, in differing amounts, as boron-bonded halogen in the addition product and as boron trihalide. 

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