Allotropes of boron

Boron (B), the second hardest element, is the only allotropic element in Group 13. It is second only to carbon (C) in its ability to form element-element bonded networks. Thus, in addition to amorphous boron, several different allotropes of boron are known, of which three are well characterized. These are red crystalline a-rhombohedral boron, black crystalline /3-rhombohedral boron (the most thermodynamically stable allotrope), and black crystalline /3-tetragonal boron. All are polymeric and are based on various modes of condensation of the Bj2 icosahedron (Figure 2). 

The a-form consists of a cubic close-packed arrangement of icosahe-dra which are linked by bonds in which three boron atoms and two electrons participate. If three boron atoms supply one orbital each, the orbitals form three molecular orbitals, one of which is bonding and can aax>mmodate the two available electrons. In spite of the relatively weak bonding between the icosahedral units, the three bonding electrons per atom bestow sufficient cohesion on the various allotropes of boron to make the enthalpy of atomization considerably greater than that of beryllium. 

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... 

This section will focus on homonuclear neutral or anionic clusters of the elements aluminum, gallium, indium, and thallium, which have an equal number of cluster atoms and substituents. Thus, they may clearly be distinguished from the metalloid clusters described below, which in some cases have structures closely related to the allotropes of the elements and in which the number of the cluster atoms exceeds the number of substituents. The compounds described here possess only a single non-centered shell of metal atoms. With few exceptions, their structures resemble those of the well-known deltahedral boron compounds such as B4(CMe3)4 [30], B9CI9 [31] or [B H ]2 [32]. The oxidation numbers of the elements in these... 

A typical building block used to construct several solid-state structures (boron-rich borides and allotropes of elemental boron) is the B12 icosahedron. According to King, (1993) an icosahedral B12 building block in which each of the 12 vertices contributes a single electron for an external two-electron two-centre (2e, 2c) bond to an external group implies the following electron count ... 

Several allotropic forms of boron are known which are based on various ways of joining B12 icosahedra (using the external orbitals on each boron atom). 

The electron-deficient character of boron also affects its allotropic forms. The high ionization energies and small size prevent boron from adopting... 

The crystal structures of boron are complex. As many as 16 distinct allotropes have been reported, but some have been poorly characterized. Eight of these have been studied as single crystals and others as... 

The a-rhombohedral form of boron (a-boron) was first reported by L. V. McCarty, J. S. Kasper, F. H. Horn, B. F. Decker, and A. E. Newkirk.1 Of the many allotropic forms of boron, it has the simplest structure.2 It may be prepared by the pyrolysis of boron(III) iodide on a tantalum filament at 800-1000°C., but the product is usually contaminated by other allotropic varieties of boron.1,4 Recently, Hagenmuller and Naslain showed that boron(III) bromide may be reduced by... 

The uniqueness of boron is clearly seen in its elemental forms, the number and structural complexity of which exceed those of any other element. At least five distinct allotropes are known, all of which contain icosahedral B12 cluster units that in most cases are accompanied by other boron atoms lying outside the icosahedral cages. The most thermodynamically stable form, j8-rhombohedral boron, has 105 B atoms in its unit cell, while the /3-tetragonal phase has 192 atoms and is still not completely elucidated despite years of study ... 

Boron s chemistry is so different from that of the other elements in this group that it deserves separate discussion. Chemically, boron is a nonmetal in its tendency to form covalent bonds, it shares more similarities with carbon and silicon than with aluminum and the other Group 13 elements. Like carbon, boron forms many hydrides like silicon, it forms oxygen-containing minerals with complex structures (borates). Compounds of boron have been used since ancient times in the preparation of glazes and borosilicate glasses, but the element itself has proven extremely difficult to purify. The pure element has a wide diversity of allotropes (different forms of the pure element), many of which are based on the icosahedral Bj2 unit. 

One of the unusual properties of boron is the many physical forms, called allotropes, in which it occurs. Allotropes are forms of an element with different physical and chemical properties. One form of boron consists of clear red crystals with a density of 2.46 grams per cubic centimeter. A second form consists of black crystals with a metallic appearance and a density of 2.31 grams per cubic centimeter. Boron can also occur as a brown powder with no crystalline stmcmre. The density of this powder is 2.350 grams per cubic centimeter. 

Boron of low purity is obtained by reduction of the oxide by Mg, followed by washing the product with alkali, hydrochloric acid and then hydrofluoric acid. The product is a very hard, black solid of low electrical conductivity which is inert towards most acids, but is slowly attacked by concentrated HNO3 or fused alkali. Pure boron is made by the vapour-phase reduction of BBr3 with H2, or by pyrolysis of B2H6 or BI3. At least four allotropes can be obtained under different conditions but transitions between them are extremely slow. For a discussion of the production of boron fibres, see Section 27.7. 

Elements. Those elements that form extended covalent (as opposed to metallic) arrays are boron, all the Group IV elements except lead, also phosphorus, arsenic, selenium and tellurium. All other elements form either only metallic phases or only molecular ones. Some of the above elements, of course, have allotropes of metallic or molecular type in addition to the phase or phases that are extended covalent arrays. For example, tin has a metallic allotrope (white tin) in addition to that with the diamond structure (grey tin), and selenium forms two molecular allotropes containing Se8 rings, isostruc-... 

The propensity of boron to form polyhedral structures is reflected also in the structures of elemental boron and boron-rich metal borides. In hydrocarbon chemistry, benzene is characterized by its extra stability the thermodynamically most stable allotrope of carbon, namely, graphite is formed by the condensation of benzene units. This beautiful relationship between compounds and allotropes exists in boron chemistry as well, where the stable allotropes of elemental boron and many of the boron-rich metal borides are made up of icosahedral subunits. 

F. R. Corrigan and F. P. Bundy, Direct transition among allotropic forms of boron nitride at high pressures and temperatures, J. Chem. Phys. 1975, 63, 3812-3820. 

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