Reactions and Compounds of Boron

Elemental boron is a semimetal that is remarkably unreactive; in contrast, the other group 13 elements all exhibit metallic properties and reactivity. We therefore consider the reactions and compounds of boron separately from those of other elements in the group. All group 13 elements have fewer valence electrons than valence orbitals, which generally results in delocalized, metallic bonding. With its high ionization energy, low electron affinity, low electronegativity, and small size, however, boron does not form a metallic lattice with delocalized valence electrons. Instead, boron forms unique and intricate structures that contain multicenter bonds, in which a pair of electrons holds together three or more atoms.

Figure 1 : Solid Boron Contains B₁₂ Icosahedra. Unlike metallic solids, elemental boron consists of a regular array of B₁₂ icosahedra rather than individual boron atoms. Note that each boron atom in the B₁₂ icosahedron is connected to five other boron atoms within the B₁₂ unit. (a) The allotrope of boron with the simplest structure is α-rhombohedral boron, which consists of B₁₂ octahedra in an almost cubic close-packed lattice. (b) A side view of the structure shows that icosahedra do not pack as efficiently as spheres, making the density of solid boron less than expected.

Elemental boron forms multicenter bonds, whereas the other group 13 elements exhibit metallic bonding.

The basic building block of elemental boron is not the individual boron atom, as would be the case in a metal, but rather the B₁₂ icosahedron. Because these icosahedra do not pack together very well, the structure of solid boron contains voids, resulting in its low density (Figure 1

). Elemental boron can be induced to react with many nonmetallic elements to give binary compounds that have a variety of applications. For example, plates of boron carbide (B₄C) can stop a 30-caliber, armor-piercing bullet, yet they weigh 10%–30% less than conventional armor. Other important compounds of boron with nonmetals include boron nitride (BN), which is produced by heating boron with excess nitrogen (Equation 4); boron oxide (B₂O₃), which is formed when boron is heated with excess oxygen (Equation 5); and the boron trihalides (BX₃), which are formed by heating boron with excess halogen (Equation 6 ).

Boron nitride is similar in many ways to elemental carbon. With eight electrons, the B–N unit is isoelectronic with the C–C unit, and B and N have the same average size and electronegativity as C. The most stable form of BN is similar to graphite, containing six-membered B₃N₃ rings arranged in layers. At high temperature and pressure, hexagonal BN converts to a cubic structure similar to diamond, which is one of the hardest substances known. Boron oxide (B₂O₃) contains layers of trigonal planar BO₃ groups (analogous to BX₃) in which the oxygen atoms bridge two boron atoms. It dissolves many metal and nonmetal oxides, including SiO₂, to give a wide range of commercially important borosilicate glasses. A small amount of CoO gives the deep blue color characteristic of “cobalt blue” glass.

At high temperatures, boron also reacts with virtually all metals to give metal borides that contain regular three-dimensional networks, or clusters, of boron atoms. The structures of two metal borides—ScB₁₂ and CaB₆—are shown in Figure 2 . Because metal-rich borides such as ZrB₂ and TiB₂ are hard and corrosion resistant even at high temperatures, they are used in applications such as turbine blades and rocket nozzles.

Figure 2 : The Structures of ScB₁₂and CaB₆, Two Boron-Rich Metal Borides. (a) The structure of ScB₁₂ consists of B₁₂ clusters and Sc atoms arranged in a faced-centered cubic lattice similar to that of NaCl, with B₁₂units occupying the anion positions and scandium atoms the cation positions. The B12 units here are not icosahedra but cubooctahedra, with alternating square and triangular faces. (b) The structure of CaB₆ consists of octahedral B₆ clusters and calcium atoms arranged in a body-centered cubic lattice similar to that of CsCl, with B₆ units occupying the anion positions and calcium atoms the cation positions.

Boron hydrides were not discovered until the early 20th century, when the German chemist Alfred Stock undertook a systematic investigation of the binary compounds of boron and hydrogen, although binary hydrides of carbon, nitrogen, oxygen, and fluorine have been known since the 18th century. Between 1912 and 1936, Stock oversaw the preparation of a series of boron–hydrogen compounds with unprecedented structures that could not be explained with simple bonding theories. All these compounds contain multicenter bonds. The simplest example is diborane (B₂H₆), which contains two bridging hydrogen atoms (part (a) in Figure 3 . An extraordinary variety of polyhedral boron–hydrogen clusters is now known; one example is the B₁₂H₁₂²⁻ ion, which has a polyhedral structure similar to the icosahedral B₁₂ unit of elemental boron, with a single hydrogen atom bonded to each boron atom.

Figure 3 : The Structures of Diborane (B₂H₆) and Aluminum Chloride (Al₂Cl₆). (a) The hydrogen-bridged dimer B₂H₆ contains two three-center, two-electron bonds as described for the B₂H₇⁻ ion in Figure 21.5. (b) In contrast, the bonding in the halogen-bridged dimer Al₂Cl₆ can be described in terms of electron-pair bonds, in which a chlorine atom bonded to one aluminum atom acts as a Lewis base by donating a lone pair of electrons to another aluminum atom, which acts as a Lewis acid.

A related class of polyhedral clusters, the carboranes, contain both CH and BH units; an example is shown here. Replacing the hydrogen atoms bonded to carbon with organic groups produces substances with novel properties, some of which are currently being investigated for their use as liquid crystals and in cancer chemotherapy.

The enthalpy of combustion of diborane (B₂H₆) is −2165 kJ/mol, one of the highest values known:

Consequently, the US military explored using boron hydrides as rocket fuels in the 1950s and 1960s. This effort was eventually abandoned because boron hydrides are unstable, costly, and toxic, and, most important, B₂O₃ proved to be highly abrasive to rocket nozzles. Reactions carried out during this investigation, however, showed that boron hydrides exhibit unusual reactivity.

Because boron and hydrogen have almost identical electronegativities, the reactions of boron hydrides are dictated by minor differences in the distribution of electron density in a given compound. In general, two distinct types of reaction are observed: electron-rich species such as the BH₄⁻ ion are reductants, whereas electron-deficient species such as B₂H₆ act as oxidants.