Tetraborane (systematically named ''arachno''-tetraborane(10)) was the first

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...

compound to be classified by Stock and Messenez in 1912 and was first isolated by Alfred Stock. It has a relatively low boiling point at 18 °C and is a gas at room temperature. Tetraborane gas is foul smelling and toxic.

History

The class of

boranes Boranes is the name given to compounds with the formula BxHy and related anions. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bond ...

was elucidated using

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

analysis by Lipscomb et al. in the 1950s. The X-ray data indicated two-electron multicenter bonds. Later, analysis based on high-resolution X-ray data was performed to analyze the

charge density In electromagnetism, charge density is the amount of electric charge per unit length, surface area, or volume. Volume charge density (symbolized by the Greek letter ρ) is the quantity of charge per unit volume, measured in the SI system in co ...

Structure

Like other

, the structure of tetraborane involves multicenter bonding, with hydrogen bridges or protonated double bonds. According to its formula, B₄H₁₀, it is classified as an ''arachno''-cluster and has a butterfly geometry, which can be rationalized by Wade's rules. Each boron is ''sp³'' hybridized, and “the configuration of the three hydrogens surrounding borons B1 and B3 is approximately trigonal and suggests approximately tetrahedral hybridization for these borons which would predict bond angles of 120°.”Lipscomb, William N. ''Boron Hydrides''. New York: W. A. Benjamin, 1963. Print. However, the boron arrangements can be classified as fragments of either the icosahedron or the octahedron because the bond angles are actually between 105° and 90°. The comparison of the diffraction data from X-ray diffraction and electron diffraction gave suspected bond lengths and angles: B1—B2 = 1.84 Å, B1—B3= 1.71 Å, B2—B1—B4= 98 ̊, B—H = 1.19 Å, B1—Hμ = 1.33 Å, B2—Hμ =1.43 Å.

Preparation

Tetraborane can be produced via a reaction between acid and magnesium, aluminum, or beryllium borides. Hydrolysis of magnesium boride,

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a Catalysis, catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or S ...

of boron halides at high temperatures and the pyrolysis of

diborane Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...

also produce tetraborane. The hydrolysis of magnesium boride was one of the first reactions to give a high yield (14%) of tetraborane. Phosphoric acid proved to be the most efficient acid (other than hydrochloric and sulfuric acid) in the reaction with magnesium boride.

Isomers

Scientists are currently working to produce the bis(diboranyl) isomer of the ''arachno''-tetraborane structure. The bis(diboranyl) is expected to have a lower energy at the Hartree-Fock method (HF) level. There is some evidence that the bis(diboranyl) isomer is initially produced when synthesizing tetraborane by the

Wurtz reaction In organic chemistry, the Wurtz reaction, named after Charles Adolphe Wurtz, is a coupling reaction whereby two alkyl halides are treated with sodium metal to form a higher alkane. : 2 R−X + 2 Na → R−R + 2 NaX The reaction is of little v ...

or coupling of B₂H₅I in the presence of

sodium amalgam Sodium amalgam, commonly denoted Na(Hg), is an alloy of mercury and sodium. The term amalgam is used for alloys, intermetallic compounds, and solutions (both solid solutions and liquid solutions) involving mercury as a major component. Sodium amal ...

. Three pathways of conversion from the bis(diboranyl) isomer into the ''arachno''-tetraborane structure have been constructed computationally. :Path 1: Dissociative pathway via B₃H₇ and BH₃ :Path 2: Concerted pathway over two transition states separated by a local minimum :Path 3: Another concerted pathway involving penta-coordinated isomers as intermediates Paths 2 and 3 are more likely, because they are more energetically favored with energies of 33.1 kcal/mol and 22.7 kcal/mol respectively.

Safety

Because it is easily oxidized it must be kept under vacuum. Tetraborane ignites when it comes in contact with air, oxygen, and nitric acid.

Boranes Boranes is the name given to compounds with the formula BxHy and related anions. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bond ...

in general including tetraborane have been deemed very toxic and are biologically destructive. A study consisting of small daily exposure of the chemical to rabbits and rats resulted in fatality.

References

External links

* * {{Hydrides by group Boranes Foul-smelling chemicals