Boron: Inorganic Chemistry

Abstract

Boron is unique among the elements and displays remarkable chemistry in all of its compounds. It is widely distributed at low concentrations in the environment and is nearly always found bound to oxygen in its natural forms. Boron is roughly the tenth most abundant element in seawater, where it exists primarily as boric acid. In addition to amorphous boron, several crystalline allotropes exist, with the β‐rhombohedral phase being the most stable form of the element under standard conditions. The borospherene B 40 cluster and 2D borophenes are more recently documented forms of boron. Metal borides are important to electronics, thermoelectrics, and superhard materials. Most have high melting points, high hardnesses, and good resistance to chemical attack. Refractory boron materials, including boron carbides, nitrides, and phosphides are discussed, as well as molecular boron–nitrogen, boron–phosphorus, boron–sulfur, and boron halide compounds. Boron‐containing amorphous metals are used as master alloys in electrical devices and in strong magnets. The vast majority of industrial use of boron on a tonnage basis involves boron–oxygen compounds. Condensation of boric acid, B(OH) 3 , and its conjugate base, B(OH) 4 − , in aqueous solution results in polyborate anions which provide the basis for many industrial products. Boric acid reacts with alcohols to form esters having industrial uses. Reversible borate ester formation with 1,2‐ and 1,3‐diols provides the basis for adhesives and oilfield applications and is central to the bioinorganic chemistry of boron. Boron enters the life cycle of plants and animals, primarily as naturally occurring boric acid found at low concentrations in natural waters and soils. Boron is essential to plants, including important food crops, resulting in extensive use of borates as agricultural micronutrients to correct suboptimal soil boron levels. Borax pentahydrate (disodium tetraborate pentahydrate) is the world's largest volume boron chemical product, followed by boric acid. Major industrial applications of borates include the manufacture of glasses, ceramics, electronic displays, consumer products, construction materials, industrial fluids, fire retardants and in metallurgy, optoelectronics, and agriculture. Other commercially important compounds, such as zinc borates, are discussed.