Boron compounds

These minerals resemble silicates in some respect, although boron is often found not only in a tetrahedral coordination with oxygen, but also in a trigonal planar configuration.

A typical motif is exemplified by the tetraborate anions of the common mineral borax, shown at left.

The formal negative charge of the tetrahedral borate center is balanced by metal cations in the minerals, such as the sodium (Na+) in borax.

The parent member BH3 is called borane, but it is known only in the gaseous state, and dimerises to form diborane, B2H6.

The formal oxidation number in boranes is positive, and is based on the assumption that hydrogen is counted as −1 as in active metal hydrides.

In the diamond-like structure, called cubic boron nitride (tradename Borazon), boron atoms exist in the tetrahedral structure of carbon atoms in diamond, but one in every four B-N bonds can be viewed as a coordinate covalent bond, wherein two electrons are donated by the nitrogen atom which acts as the Lewis base to a bond to the Lewis acidic boron(III) centre.

Cubic boron nitride, among other applications, is used as an abrasive, as it has a hardness comparable with diamond (the two substances are able to produce scratches on each other).

Organoboron chemicals have been employed in uses as diverse as boron carbide (see below), a complex very hard ceramic composed of boron-carbon cluster anions and cations, to carboranes, carbon-boron cluster chemistry compounds that can be halogenated to form reactive structures including carborane acid, a superacid.

As anticipated by its hydride clusters, boron forms a variety of stable compounds with formal oxidation state less than three.

[6] Binary metal-boron compounds, the metal borides, contain boron in negative oxidation states.

A project at CERN to make MgB2 cables has resulted in superconducting test cables able to carry 20,000 amperes for extremely high current distribution applications, such as the contemplated high luminosity version of the large hadron collider.