Covalent hydrides boranes

As we shall see later, borides (as well as oxides, nitrides, carbides, etc.) react with water to produce a hydrogen compound of the nonmetal. Thus, the reaction of magnesium boride with water might be expected to produce BH3, borane, but instead the product is B2ff6, diborane (m.p. -165.5 °C, b.p. -92.5 °C). This interesting covalent hydride has the structure... 

Boron and hydrogen form many compounds and they exhibit unusual structural forms. Several of the boranes are listed in Table 13.2. Covalent hydrides are generally compounds that have low boiling points. Consequently, they are often referred to as volatile hydrides. 

Boron is a typical nonmetal, and most of its compounds are covalent. The most interesting compounds of boron are the covalent hydrides called boranes. We might expect BH3 to be the simplest hydride, since boron has three valence electrons to share with three hydrogen atoms. However, this compound is unstable, and the simplest known member of the series is diborane (B2H6), with the structure shown in Fig. 20.7(a). hi this... 

While the connection between the structure of the hydrides and their pyrophoric behavior seems to be obscure, it should be pointed out that the compounds of hydrogen fall into three distinctive groups the above-described phosphines, silanes, and boranes, which have covalent bonds the salt-like hydrides of the alkali and alkaline earth metals and the interstitial, nonstoicbiometric or berthollide —type hydrides of the transition metals—e.g. the rare earths, titanium, and zirconium. Pyrophoric compounds are found in all three groups. 

Despite these spectacular successes, the molecular structures of the boranes were entirely unknown and basically unknowable at that time. Lacking modem tools such as NMR and single-crystal X-ray diffi-action techniques (5), and with ideas of covalence in that era dominated by Lewis-style electron pair bonding as found in organic compounds. Stock and his contemporaries could only speculate about structure. For lack of a better idea, they assumed that the boron hydrides, notwithstanding an apparent deficiency of electrons, must adopt hydrocarbon-like chain structures such as the examples shown in Chart 1, for which both non-ionic and ionic models were suggested (4). 

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