Elementary Boron

Other boron atoms each contribute two-thirds of an electron. All together 36 electrons per icosahedron (the 3 x 12 valence electrons of the 12 boron atoms) are involved in a-rhombohedral boron bonding. 

The third modification is a-tetragonal boron, which is based on four icosahedra per unit cell, which are bound to each other either directly or through two single boron atoms. This allotrope has the structure of many binary boron-rich compounds of nonmetals. 

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Elementary boron exists in the solid state in several allotropic forms. Those which have been properly characterized by X-ray diffraction contain icosahedral B12 units, which are also the basis of many boron cluster compounds. 

B5H9 the apex boron forms foqr B—B bonds with bond number 0.64). The conclusion may be drawn that the structural feature leading to greatest stability is ligancy 6 with one nonbridging B—H bond (or B—B bond, as in elementary boron). 

When elementary boron and boron compounds such as boric acid,... 

Figure 4 Boron clusters occurring in boron-rich borides and elementary boron (a) octahedron (b) cubooctahedron (c) icosahedron (d) the main part of a Bs4 unit (/3-rh. boron) and an attached Bio unit (black circles) with two half-icosahedra (hatched circles)...

There are very few examples as yet of simple inorganic compounds with structures based on 3-dimensional 3-connected nets. We might have expected to find examples among the crystalline compounds of boron, an element which forms three coplanar bonds in many simple molecules and ions. The structure of the normal form of B2O3 is in fact based on a simple 3-connected net however, boron is 4-coordinated in many borates, and both triangular and tetrahedral coordination occur in many compounds. The complex crystalline forms of elementary boron are not simple covalent structures but electron-deficient systems of a quite special kind, the structures of which are briefly described in Chapter 24. 

Ozawa et al. also performed similar experimraits on boron compounds by bombarding with nitrogen ions (N" ). The measured spectrum is shown in Fig. 7. Satellite lines such as KL and K L appear in the case of boron nitride, while they do not appear in the case of elementary boron. The shoulder marked by K is ascribed to transition of an electron of N(2s) to a vacancy of B(ls). 

Fig. 7. K X-ray spectra for elementary boron (dashed line) and boron nitride (solid line)lx>mbarded...

Apart from the diborides, also some monoborides are technically important. Most of them are used as hard components in composite materials. One notable compound is FeB. It has a remarkable adhesiveness on iron-based materials and thus increases their hardness. Such hard coatings are produced via CVD techniques. As an intermediate product elementary boron is obtained through the reaction 2BCI3 -I- 3H2 — 2B -1- 6HC1 between 500 and 900°C, which subsequently reacts with iron to form the FeB coating [32,34]. 

Elementary boron has several modifications. Figure 4.7 shows the structure of a-rhombohedral boron. Each boron atom is linked with covalent bonds to six or seven neighboring boron atoms. The slightly distorted icosahedra are in what is approximately a cubic close-packed arrangement. Six of the twelve boron atoms in one icosahedron are bound with a three-center, two-electron bond to two other icosahedra simultaneously, and the other six are bound to one other icosahedron with a conventional covalent two-center, two-electron bond. Six boron atoms contribute one electron each to the common bond between the icosahedra, the six... 

B10H14 and some other borane molecules have also been found to have structures based on the icosahedron, with some of the corners not occupied by boron atoms, and with some bridging hydrogen atoms, as in diborane. The B12 icosahedron is also present in elementary boron and in the hard substance B4C. 

Structures of Boranes and Carboranes. As mentioned above, elementary boron occurs in its most stable modification as B12 icosahedric units connected to each other by six boron-boron two-center bonds. Boron-boron distances within the icosahedron range from 1.73 to 1.79 A. From a structural point of view, boranes can be systematized by considering them as derivatives of the elementary icosahedron, but built by B-H units. Some examples of geometrical forms that can be derived from icosahedron were shown in Fig. 2.27. 

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