Boron hydrides bonding

The number of species with incomplete octets is limited to some beryllium, boron, and aluminum compounds. Perhaps the best examples are the boron hydrides. Bonding in the boron hydrides will be discussed in Chapter 22. 

Hydroboration is a reaction m which a boron hydride a compound of the type R2BH adds to a carbon-carbon bond A carbon-hydrogen bond and a carbon-boron bond result... 

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. 

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. 

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

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. 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

Hydroboration, discovered by Herbert C. Brown of Purdue University (co-winner of the Nobel Prize for Chemistry in 1979), involves an addition of a H-B bond (a boron hydride) to an alkene. 

Bonding of this type and other boron hydrides that have three-center two-electron bonds with hydrogen bridges is discussed in Chapter 13. 

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. 

The properties of some boron hydrides along with those of other volatile hydrides are shown in Table 13.2. A very interesting reaction that diborane undergoes is one in which it reacts with double bonds in hydrocarbons. The reaction can be shown as... 

There were a considerable number of reports over the period 1990-2004 on polyhedral boron hydride species with exo-chalcogen bonds and the following section is structured in terms of increasing polyhedral size. There were also a few reports on polyhedral carborane species, which are at the end of this section, but there were no reports on other heteroborane species with exo-chalcogen bonds. 

Table 3.38. Comparison of calculated (B3LYP/6-311++G ) and experimental (note 154) bond lengths and bond angles for boron hydrides (see Fig. 3.103). H( ) denotes a bridge hydrogen...

Pentaborane, B5H9, is unusual in having a tetragonal C4v skeleton, rather than the icosahedral fragment geometry that is typical of other boron hydrides. The unique apex atom B1 makes four short B—B bonds to the base (1.695 A), whereas the four basal B—B bonds are of more typical length (1.798 A). 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

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