The Boron Hydrides

If We ignore the B(ls)2 electrons we have 2 x 3(B) + 6(H) = 12 valence electrons. We assume that the B 2s and 2p orbitals are sp3-hybridized. Thus 8 electrons can be placed in four a bonds from the two borons to Ha, H, H, and Hg. This means that we have 4 

The symmetry operations of the molecule place it in the point group D2h. The D2h character table is given in Table 7-1. 

In the next chapter we shall discuss molecular orbital theory as applied to transition metal complexes. Since we shall be dealing with d orbitals, the power of the symmetry methods developed up to this point will be clearly shown. 

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Boron s electron deficiency does not permit conventional two-electron bonds. Boron can form multicenter bonds. Thus the boron hydrides have stmctures quite unlike hydrocarbons. The B nucleus, which has a spin of 3/2, which has been employed in boron nuclear magnetic resonance spectroscopy. 

The hydrides of the later main-group elements present few problems of classification and are best discussed during the detailed treatment of the individual elements. Many of these hydrides are covalent, molecular species, though association via H bonding sometimes occurs, as already noted (p. 53). Catenation flourishes in Group 14 and the complexities of the boron hydrides merit special attention (p. 151). The hydrides of aluminium, gallium, zinc (and beryllium) tend to be more extensively associated via M-H-M bonds, but their characterization and detailed structural elucidation has proved extremely difficult. 

The structural complexity of borate minerals (p. 205) is surpassed only by that of silicate minerals (p. 347). Even more complex are the structures of the metal borides and the various allotropic modifications of boron itself. These factors, together with the unique structural and bonding problems of the boron hydrides, dictate that boron should be treated in a separate chapter. 

It is also noteworthy that Alfred Stock, who is universally acclaimed as the discoverer of the boron hydrides (1912). " was also the first to propose the use of the term "ligand (in a lecture in Berlin on 27 November 1916). Both events essentially predate the formulation by G. N. Lewis of the electronic theory of valency (1916). It is therefore felicitous that, albeit some 20 years after Stock s death in 1946, two such apparently disparate aspects of his work should be connected in the emerging concept of boranes as ligands . 

The boron hydrides offer an interesting problem. With boron trivalent, we should expect compounds such as BF3, BH3, etc. 

E O Fischer and H. P. Fritz Recent Studies of the Boron Hydrides William A, Lipscomb Lattice Energies and Their Significance in Inorganic Chemistry T C. Waddington... 

Hydroboration can be carried out by using the boron hydride (B2H6) called diborane. 

The boron hydrides, silanes, phosphines, and most other covalent hydrides bum readily, some even being spontaneously flammable. 

In these molecules, the boron atom has only six electrons surrounding it, so it interacts readily with species that can function as electron pair donors. For example, when l reacts with BF3, the product is BF4-, in which sp3 hybrids are formed, so such species are tetrahedral (7 ( symmetry). In most cases, molecules containing boron exhibit one of these types of bonding to boron. The boron hydrides represent a special situation that is described later. 

There are many compounds that contain boron and hydrogen, and they are known collectively as the boron hydrides. Six boron hydrides were prepared by Alfred Stock in 1910-1930 by the addition of hydrochloric acid to magnesium boride that was produced in small amounts when B203 was reduced with magnesium. 

Since the early work of Stock, other boron hydrides have been synthesized. Some of these compounds have been used as fuel additives, and they have found some application in high-energy rocket fuels. However, because B203(s) is a product of the reaction, the use of these materials in that way causes some problems. The boron hydrides will all bum readily to produce B203 and water,... 

General references W. N. Lipscomb, Recent Studies of the Boron Hydrides, in Adv. Inorg. Radiochem., 1959, 3, 117 Boron Hydride Chemistry, Academic Press, New York, 1975 E. L. Muetterties, W. H. Knoth,... 

One of the main drawbacks to the development of the chemistry of many of the boron hydrides has been the absence of synthetic procedures for producing these materials in reasonable yields and quantities by relatively safe and simple techniques. Classical approaches are heavily dependent upon pyrolytic procedures. Although they have been developed to a "fine art," they require a high degree of skill in order to be employed safely in ordinary laboratory environments. Other important classical methods are dependent upon controlled protolysis reactions, frequently giving mixtures of materials which are difficult to separate. 

Many of the boron hydrides have been shown to function as Brdnsted acids (See 34 for references). The bridging hydrogens are acidic. Pentaborane(9) is a monoprotic acid in the presence of a variety of bases (35,36,37). An example of such a reaction (38) is given in Scheme I, Reaction (4). By removing the bridging proton to give BsHs , a boron-boron bond is formed which is susceptible to insertion of electrophillic agents. 

Recent Studies of the Boron Hydrides William N. Lipscomb... 

Chemical Reactivity of the Boron Hydrides and Related Compounds... 

Ach on carbon orbital hybridization in hydrocarbons is not found to hold quantitatively for the boron hydrides. 

The boron hydrides, including the polyhedral boranes, heteroboranes, and their metaUa derivatives, encompass an amazingly diverse area of chemistry. This class contains the most extensive array of structurally characterized cluster compounds known. Included here are many novel clusters possessing idealized molecular geometries ranging over every point group symmetry from identity (C[) to icosahedral (I[). Because boron hydride clusters may be considered in some respects to be progenitorial models of metal clusters, their development has provided a framework for the development of cluster chemistry in... 

Diborane is the simplest of the boron hydrides, a class of compounds that have become known as electron deficient They are electron deficient only in a formal sense—there are fewer electrons than required for all of the adjacent atoms to be held together by electron-pair covalent bonds. The compounds, in feet, are good reducing... 

But carbon is not unique in forming bonds to itself because other elements such as boron, silicon, and phosphorus form strong bonds in the elementary state. The uniqueness of carbon stems more from the fact that it forms strong carbon-carbon bonds that also are strong when in combination with other elements. For example, the combination of hydrogen with carbon affords a remarkable variety of carbon hydrides, or hydrocarbons as they usually are called. In contrast, none of the other second-row elements except boron gives a very extensive system of stable hydrides, and most of the boron hydrides are much more reactive than hydrocarbons, especially to water and air. 

Some transition-metal hydrides show promise as synthetic reagents of the same general applicability as the boron hydrides (Section 11-6). An excellent illustration is provided by the work of J. Schwartz with zirconocene chloro-... 

Boron Hydrides The boron hydrides, or boranes, are volatile, molecular compounds with formulas B Hm. The simplest is diborane (B2H6), the dimer of the unstable BH3. Diborane can be prepared by the reaction of sodium borohydride (NaBH4) and iodine in an appropriate organic solvent ... 

Its unique reactivity comes from the fact that borane first forms a Lewis acid-base complex with the acid and then a boron-carboxylate intermediate which increases the reactivity of the boron hydride and delivers the hydride by an intramolecular reaction. As such it provides a selective way to reduce acids and produce alcohols in the presence of most other functional groups. 

The closodecaborate anion is part of the inner sphere in complexes 562 too [960]. The examined anion acts as a counterion in complex compounds, containing the substituents (R - SMe2 [961], CH2CN [962]) in the boron-hydride fragment. The complexes of dodecahydroclosododecaborate anion have similar structures [964,965], for example 563 ... 

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