In chemistry, a
borane is a chemical compound of
boronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite....
and
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
. The boranes comprise a large group of compounds with the generic formulae of B
xH
y. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The parent member BH
3 is called borane, but it is known only in the gaseous state, and
dimerA dimer is a chemical or biological entity consisting of two structurally similar subunits called monomers, which are joined by bonds, which can be strong or weak.- Organic chemistry :...
ises to form
diboraneDiborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air...
, B
2H
6. The larger boranes all consist of boron
clustersIn chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
that are polyhedral, some of which exist as
isomerIn chemistry, isomers are compounds with the same molecular formula but different structural formulas. The word is derived from the Greek ισομερης, isomerès; isos = "equal", méros = "part"....
s. For example, isomers of B
20H
26 are based on the fusion of two 10-atom clusters.
The most important boranes are
diboraneDiborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air...
B
2H
6,
pentaboranePentaborane, also called pentaboron nonahydride, stable pentaborane, or pentaborane , is a chemical compound considered in the 1950s as a good prospect for a rocket or jet fuel by both the U.S. and Russian armed services, a so-called "exotic fuel"...
B
5H
9, and
decaboraneDecaborane, also called decaborane, is the borane with the chemical formula B10H14. This white crystalline compound is one of the principal boron hydride clusters, both as a reference structure and as a precursor to other boron hydrides.-Handling properties and...
B
10H
14.
The development of the chemistry of boron hydrides led to new experimental techniques and theoretical concepts. Boron hydrides have been studied as potential fuels, for rockets and for automotive uses.
Over the past several decades, the scope of boron hydride chemistry has grown to include cages containing atoms other than boron, such as carbon in the
carboraneA carborane is a cluster composed of boron and carbon atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc...
s and metals in the metallaboranes, wherein one or more boron atoms are substituted by metal atoms.
Generic formulae of boranes
The four series of single-cluster boranes have the following general formulae, where "n" is the number of boron atoms:-
| Type |
formula |
notes |
| closo− |
BnHn2− |
No neutral BnHn+2 boranes are known |
| nido− |
BnHn+4 |
|
| arachno− |
BnHn+6 |
|
| hypho− |
BnHn+8 |
only adducts established |
There also exists a series of substituted neutral
hypercloso-boranes that have the theoretical formulae B
nH
n. Examples include B
12(OCH2Ph)
12, which is a stable derivative of
hypercloso-B
12H
12.
Naming conventions
The naming of neutral boranes is illustrated by the following examples, where the Greek prefix shows the number of boron atoms and the number of hydrogen atoms is in brackets:-
- B5H9 pentaborane(9)
- B6H12 hexaborane(12)
The naming of anions is illustrated by the following, where the hydrogen count is specified first followed by the boron count, and finally the overall charge in brackets:-
- B5H8− octahydropentaborate(1−)
Optionally
closo− nido− etc (see above) can be added:-
- B5H9, nido−pentaborane(9)
- B4H10
Tetraborane, or to be more precise tetraborane or arachno-B4H10, is a foul-smelling toxic gas. The structure of tetraborane is based around a tetrahedral arrangement of boron atoms, and can be derived from PSEPT. It forms interesting coordinate bonds with...
, arachno−tetraborane(10)
- B6H62−, hexahydro−closo−hexaborate(2−)
Understandably many of the compounds have abbreviated common names.
Cluster types
It was realised in the early 1970s that the geometry of boron clusters are related and that they approximate to deltahedra or to deltahedra with one or more vertices missing. The
deltahedraA deltahedron is a polyhedron whose faces are all equilateral triangles. The name is taken from the Greek majuscule delta , which has the shape of an equilateral triangle. There are infinitely many deltahedra, but of these only eight are convex, having 4, 6, 8, 10, 12, 14, 16 and 20 faces...
that are found in borane chemistry are (using the names favoured by most chemists):--
| deltahedron |
vertices |
| Trigonal bipyramid |
5 |
| Octahedron |
6 |
| Pentagonal bipyramid |
7 |
| Dodecahedron (see Snub disphenoid In geometry, the snub disphenoid is one of the Johnson solids . It is a three-dimensional solid that has only equilateral triangles as faces, and is therefore a deltahedron. It is not a regular polyhedron because some vertices have four faces and others have five...
) |
8 |
| Tricapped trigonal prism |
9 |
| Bicapped square antiprism |
10 |
| Octadecahedron |
11 |
| Icosahedron |
12 |
One feature of these
deltahedraA deltahedron is a polyhedron whose faces are all equilateral triangles. The name is taken from the Greek majuscule delta , which has the shape of an equilateral triangle. There are infinitely many deltahedra, but of these only eight are convex, having 4, 6, 8, 10, 12, 14, 16 and 20 faces...
is that boron atoms at the vertices may have different numbers of boron atoms as near neighbours. For example, in the pentagonal bipyramid, 2 borons have 3 neighbors, 3 have 4 neighbours, whereas in the octahedral cluster all vertices are the same, each boron having 4 neighbours. These differences between the boron atoms in different positions are important in determining structure, as they have different chemical shifts in the
11B
NMRNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is the name given to a technique which exploits the magnetic properties of certain nuclei. This phenomenon and its origins are detailed in a separate section on nuclear magnetic resonance. The most important...
spectra.
B
6H
10 is a typical example. Its geometry is, in essence, a 7-boron framework (pentagonal bipyramid), missing a vertex that had the highest number of near neighbours, e.g., a vertex with 5 neighbours. The extra hydrogen atoms bridge around the open face. A notable exception to this general scheme is that of B
8H
12, which would be expected to have a
nido- geometry (based on B
9H
92− missing 1 vertex), but is similar in geometry to B
8H
14, which is based on B
10H
102−.
The names for the series of boranes are derived from this general scheme for the cluster geometries:-
- hypercloso- (from the Greek for "over cage") a closed complete cluster, e.g., B8Cl8 is a slightly distorted dodecahedron
- closo- (from the Greek for "cage") a closed complete cluster, e.g., icosahedral B12H122−
- nido- (from the Latin for "nest") B occupies n vertices of an n+1 deltahedron, e.g., B5H9 an octahedron missing 1 vertex
- arachno- (from the Greek for "spiders web") B occupies n vertices of an n+2 deltahedron e.g. B4H10 an octahedron missing 2 vertices
- hypho- (from the Greek for "net") B occupies n vertices of an n+3 deltahedron possibly B8H16 has this structure, an octahedron missing 3 vertices
- conjuncto- 2 or more of the above are fused together
Bonding in boranes
Boranes are
electron-deficientElectron deficiency occurs when a compound has too few valence electrons for the connections between atoms to be described as covalent bonds. Electron deficient bonds are often better described as 3-center-2-electron bonds...
and pose a problem for conventional descriptions of
covalentA covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds...
bonding that involves shared electron pairs. BH
3 is a
trigonal planarIn chemistry, trigonal planar is a molecular geometry with one atom at the center and three atoms at the corners of a triangle all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to the point group D3h...
molecule (D
3h molecular symmetryMolecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
).
DiboraneDiborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air...
has a hydrogen-bridged structure, see the
diboraneDiborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air...
article.
The description of the bonding in the larger boranes formulated by
William LipscombWilliam Nunn Lipscomb, Jr. is an American inorganic chemist, working in experimental and theoretical chemistry and biochemistry....
involved:
- 3 center 2 electron
A three-center two-electron bond is an electron deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non-bonding, and one anti-bonding. The two electrons go into the bonding orbital, resulting in a...
B-H-B hydrogen bridges
- 3-center 2-electron B-B-B bonds
- 2-center 2-electron bonds (in B-B, B-H and BH2)
The styx number was introduced to aid in electron counting where s = count of 3-center B-H-B bonds; t = count of 3-center B-B-B bonds; y = count of 2-center B-B bonds and x = count of BH
2 groups.
Lipscomb's methodology has largely been superseded by a
molecular orbitalIn chemistry, a molecular orbital is a mathematical function that describes the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The use of the term...
approach, although it still affords insights. The results of this have been summarised in a simple but powerful rule,
PSEPTIn chemistry the polyhedral skeletal electron pair theory provides electron counting rules used to predict the structure of electron deficient clusters. They were originally formulated by K. Wade and further developed by Mingos and others, and are sometimes known as Wade's Rules or Wade / Mingos...
, often known as Wade's rules, that can be used to predict the cluster type,
closo-,
nido-, etc. The power of this rule is its ease of use and general applicability to many different cluster types other than boranes.
There are continuing efforts by theoretical chemists to improve the treatment of the bonding in boranes — an example is Stone's tensor surface harmonic treatment of cluster bonding. A recent development is
four-center two-electron bondA four-center two-electron bond is a type of chemical bond in which four atoms share two electrons in bonding which is unusual because in ordinary chemical bonds two atoms share two electrons . This type of bonding is postulated in certain cluster compounds...
.
Properties and reactivity trends
Boranes are all colourless and
diamagneticDiamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...
. They are reactive compounds and some are
pyrophoricA pyrophoric substance will ignite spontaneously; that is, its autoignition temperature is below room temperature. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thinly. Pyrophoric materials are often water reactive as well and will ignite when they...
. The majority are highly poisonous and require special handling precautions.
closo−
- There is no known neutral closo borane. Salts of the closo anions, BnHn2− are stable in neutral aqueous solution, and their stabilities increase with size. The salt K2B12H12 is stable up to 700o.
nido−
- Pentaborane(9)
Pentaborane, also called pentaboron nonahydride, stable pentaborane, or pentaborane , is a chemical compound considered in the 1950s as a good prospect for a rocket or jet fuel by both the U.S. and Russian armed services, a so-called "exotic fuel"...
and decaborane(14)Decaborane, also called decaborane, is the borane with the chemical formula B10H14. This white crystalline compound is one of the principal boron hydride clusters, both as a reference structure and as a precursor to other boron hydrides.-Handling properties and...
are the most stable nido−boranes, in contrast to nido−B8H12 that decomposes above -35o.
arachno−
- Generally these are more reactive than nido−boranes and again larger compounds tend to be more stable.
Synthesis and general reactivity
Borane BH
3
- This is an important intermediate in the pyrolosis of diborane to produce higher boranes.
DiboraneDiborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air...
B
2H
6 and higher boranes
- Diborane
Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air...
is made industrially by the reduction of BF3Boron trifluoride is the chemical compound with the formula BF
3. This pungent colourless toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds.-Structure and bonding:...
, and is the starting point for preparing the higher boranes.It has been studied extensively.
General reactivity
- Typical reactions of boranes are
-
- electrophilic
In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids...
substitution
- nucleophilic substitution by Lewis bases
- deprotonation
Deprotonation is a chemistry term that refers to the removal of a proton from a molecule, forming the conjugate base. The relative ability for a molecule to give up a proton is measured by a pKa value. A low pKa value indicates that the compound is acidic and will easily...
by strong bases
- cluster building reactions with borohydrides
- reaction of a nido-borane with an alkyne
Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC...
to give a carborane clusterIn chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
- Boranes can act as ligand
In chemistry, a ligand is either an atom, ion, or molecule that binds to a central metal-atom to produce a coordination complex. The bonding between the metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The metal-ligand bonding ranges from covalent...
s in coordination compounds. HapticitiesThe term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
of η1 to η6 have been found, with electron donation involving bridging H atoms or donation from B-B bonds. For example, nido-B6H10 can replace ethene in Zeise's saltZeise's salt, potassium trichloroplatinate, is the chemical compound with the formula K
[PtCl
3]·H
2O. The anion of this air-stable, yellow, coordination complex contains an η
2-ethylene ligand. The anion features a platinum atom with a square planar...
to produce Fe(η2-B6H10)(CO)4.
Boranes can react to form hetero-boranes, e.g.,
carboraneA carborane is a cluster composed of boron and carbon atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc...
s or metalloboranes (
clustersIn chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
that contain boron and metal atoms).
History
The development of the chemistry of boranes posed two challenges to chemists. First, new laboratory techniques had to be developed to handle these very reactive compounds; second, the structures of the compounds challenged the accepted theories of chemical bonding.
The German
chemist Alfred StockAlfred Stock was a German inorganic chemist. He did pioneering research on the hydrides of boron and silicon, coordination chemistry, mercury, and mercury poisoning...
first characterized the series of boron-hydrogen compounds. His group developed the glass vacuum line and techniques for handling the compounds. However, exposure to mercury (used in mercury diffusion pumps and float valves) caused
StockAlfred Stock was a German inorganic chemist. He did pioneering research on the hydrides of boron and silicon, coordination chemistry, mercury, and mercury poisoning...
to develop mercury poisoning, which he documented in the first scientific papers on the subject. The chemical bonding of the borane
clustersIn chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
was investigated by Lipscomb and his co-workers. Lipscomb was awarded the
Nobel prizeThe Nobel Prize is a Sweden-based international monetary prize. The award was established by the 1895 will and estate of Swedish chemist and inventor Alfred Nobel. It was first awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace in 1901...
in Chemistry in 1976 for this work.
PSEPTIn chemistry the polyhedral skeletal electron pair theory provides electron counting rules used to predict the structure of electron deficient clusters. They were originally formulated by K. Wade and further developed by Mingos and others, and are sometimes known as Wade's Rules or Wade / Mingos...
, (Wades rules) can be used to predict the structures of boranes.
Interest in boranes increased during World War II due to the potential of
uranium borohydrideUranium borohydride U4 is a volatile uranium compound with boron. It is a tetrahedral monomer in its gas phase and has a vapor pressure of at 60 °C.-Preparation:...
for enrichment of the uranium isotopes. In the US, a team led by
SchlesingerHermann Irving Schlesinger was an American inorganic chemist, working in boron chemistry.He and Herbert C. Brown discovered sodium borohydride in 1940 and both were involved in further development of borohydride chemistry....
developed the basic chemistry of the boron hydrides and the related aluminium hydrides. Although uranium borohydride was not utilized for isotopic separations, Schessinger’s work laid the foundation for a host of boron hydride
reagentA reagent or reactant is a substance or compound consumed during a chemical reaction. Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants....
s for
organic synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
, most of which were developed by his student
Herbert C. BrownHerbert Charles Brown was a chemist and Nobel Prize in Chemistry laureate for his work with organoboranes....
. Borane-based reagents are now widely used in organic synthesis. For example,
sodium borohydrideSodium borohydride, also known as sodium tetrahydroborate, has the chemical formula NaBH
4. This white solid, usually encountered as a powder, is a specialty reducing agent used in the manufacture of pharmaceuticals and other organic and inorganic compounds...
is the standard reagent for converting
aldehydeAn aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom , is called the aldehyde group...
s and
ketoneIn organic chemistry, a ketone is a type of compound that features a carbonyl group bonded to two other carbon atoms, i.e., R3CCO-CR3 where R can be a variety of atoms and groups of atoms...
s to
alcoholIn chemistry, an alcohol is any organic compound in which a hydroxyl group is bound to a carbon atom of an alkyl or substituted alkyl group. An important group of acohols is formed by the simple acyclic alcohols, the general formula for which is C
nH
2n+1OH...
s. Brown was awarded the
Nobel prizeThe Nobel Prize is a Sweden-based international monetary prize. The award was established by the 1895 will and estate of Swedish chemist and inventor Alfred Nobel. It was first awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace in 1901...
in Chemistry in 1979 for this work.
In the 1950s and early '60s, the US and USSR investigated boron hydrides as high-energy fuels (ethylboranes, for example) for high speed aircraft, such as the
XB-70 ValkyrieThe North American Aviation XB-70 Valkyrie was the prototype version of the proposed B-70 nuclear-armed deep penetration bomber for the United States Air Force's Strategic Air Command...
. The development of advanced surface-to-air missiles made the fast aircraft redundant, and the fuel programs were terminated, although
triethylboraneTriethylborane , also called triethylborine and triethylboron, is an organoborane , a near-colorless to yellowish transparent liquid with pungent ether-like odor...
(TEB) was later used to ignite the engines of the
SR-71 BlackbirdThe Lockheed SR-71 is an advanced, long-range, Mach 3 strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft by the Lockheed Skunk Works as a Black project. The SR-71 was unofficially named the Blackbird, and called the Habu by its crews, referring to an Okinawan...
.
General references
- Fox M.A., Wade K. Pure Appl. Chem. (2003),75,9, 1315