Atomic boron hydrides

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 ... 

The chain-growth catalyst is prepared by dissolving two moles of nickel chloride per mole of bidentate ligand (BDL) (diphenylphosphinobenzoic acid in 1,4-butanediol). The mixture is pressurized with ethylene to 8.8 MPa (87 atm) at 40°C. Boron hydride, probably in the form of sodium borohydride, is added at a molar ratio of two borohydrides per one atom of nickel. The nickel concentration is 0.001—0.005%. The 1,4-butanediol is used to solvent-extract the nickel catalyst after the reaction. 

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. 

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... 

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. 

Table 3.40. Boron atomic charge distributions in boron hydrides see Fig. 3.103 for atom numberings), comparing Lipscomb s zeroth-order ZO) estimates note 154) with NPA atomic charges...

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). 

This chapter concerns carboranes (carbaboranes), which are boron clusters with at least one carbon atom as part of the polyhedral cage. Published studies on carboranes before 1981 were reviewed in GOMC (1982) and between 1982 and 1992 in COMC (1995). The present review covers the period of 1992 to early 2005. Unlike in previous chapters, boron hydrides with organic substituents attached to a boron atom, organopolyboron hydrides, are not discussed in this chapter. Borane clusters containing at least one non-carbon atom as part of the cage framework are reviewed in Chapters 3.03, 3.04 and 3.05 of this volume. 

Each external (i.e., terminal) B-H bond is regarded as a typical two-center two-electron single bond requiring the hydrogen Is orbital, one hybridized boron orbital, and one electron each from the H and the B atoms. Because of the small electronegativity difference between hydrogen and boron, these bonds are assumed to be non-polar. In the polynuclear boron hydrides every boron atom may form zero or one but never more than two such external B-H bonds. 

The procedures are simple and give essentially stoichiometric reactions which can be carried out in the absence of a solvent. The systematic nature of these syntheses relates to the observation that hydride ion can be abstracted from certain boron hydride anions to give as one of the final products a neutral boron hydride which contains one more boron atom than the anionic starting material. These reactions are described below. 

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