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Encyclopedia > Borane

In chemistry a borane is a chemical compound of boron and hydrogen. The boranes comprise a large group of compounds with the generic formulae of BxHy. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The smallest member BH3 is called borane, but this is only known in the gaseous state and dimerises to form diborane, B2H6. The larger members of the family have boron clusters, for example B20H26 which has a number of isomers, all of which are based on two fused 10 boron atom clusters.
The most important boranes are diborane B2H6, pentaborane B5H9 and decaborane B10H14.
Early researchers into boranes had to develop new experimental techniques. Theoretical chemists had to develop new theories of bonding to explain the structures of boranes. At one time boron compounds were studied as potential rocket fuels. Now related ranges of compounds have been discovered, e.g. carboranes where 1 or more boron atoms are substituted by carbon and metalloboranes where 1 or more boron atoms are substituted by metal atoms. Diborane is a colorless gas at room temperature with a repulsive, sweet odor. ... In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. ... Diborane is a colorless gas at room temperature with a repulsive, sweet odor. ... Pentaborane is a chemical compound considered in the 1950s as a good prospect for a rocket or jet fuel by the U.S. armed services, a so-called exotic fuel. Its chemical structure is that of five atoms of boron compounded with nine atoms of hydrogen (B5H9). ... Decaborane (B10H14) is a stable, crystalline borane. ... 3D model of carborane acid, colors: Hydrogen - white, Chlorine - yellow, Boron - green, Carbon - black A carborane is a cluster composed of boron and carbon atoms. ...

Contents

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 is a series of substituted neutral hypercloso-boranes which have the theoretical formulae BnHn. Examples include B12(OCH2Ph)12 which is a stable derivative of hypercloso-B12H12[1].


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:-
IUPAC nomenclature is a system of naming chemical compounds and of describing the science of chemistry in general. ...

  • 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:- IUPAC nomenclature is a system of naming chemical compounds and of describing the science of chemistry in general. ...

  • B5H8 octahydropentaborate(1−)


Optionally closo− nido− etc (see above) can be added:-

  • B5H9, nido−pentaborane(9)
  • B4H10, 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 1970's that the geometry of boron clusters were related and that they approximated to deltahedra or to deltahedra with one or more vertices missing.The deltahedra that are found in borane chemistry are (using the names favoured by most chemists):-- The triaugmented triangular prism, a convex deltahedron A deltahedron (plural deltahedra) is a polyhedron whose faces are all equilateral triangles. ...

deltahedron vertices
Trigonal bipyramid 5
Octahedron 6
Pentagonal bipyramid 7
Dodecahedron 8
Tricapped trigonal prism 9
Bicapped square antiprism 10
Octadecahedron 11
Icosahedron 12

One feature of these deltahedra 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 NMR spectra. The triaugmented triangular prism, a convex deltahedron A deltahedron (plural deltahedra) is a polyhedron whose faces are all equilateral triangles. ... Nuclear magnetic resonance spectroscopy most commonly known as NMR spectroscopy is the name given to the technique which exploits the magnetic properties of certain nuclei. ...

B6H10 is a typical example. Its geometry is essentally an 7 boron framework (pentagonal bipyramid) missing a vertex which 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 B8H12 which would be expected to have a nido- geometry (based on B9H92− missing 1 vertex) but is similar in geometry to B8H14 which is based on B10H102−.
Image File history File links Size of this preview: 800 × 503 pixelsFull resolution‎ (1,100 × 692 pixels, file size: 165 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Diborane is a colorless gas at room temperature with a repulsive, sweet odor. ... Image File history File links Size of this preview: 714 × 600 pixelsFull resolution‎ (1,100 × 924 pixels, file size: 287 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Pentaborane is a chemical compound considered in the 1950s as a good prospect for a rocket or jet fuel by the U.S. armed services, a so-called exotic fuel. Its chemical structure is that of five atoms of boron compounded with nine atoms of hydrogen (B5H9). ... Image File history File links Size of this preview: 763 × 600 pixelsFull resolution‎ (1,100 × 865 pixels, file size: 274 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Decaborane (B10H14) is a stable, crystalline borane. ... Image File history File links Size of this preview: 573 × 599 pixelsFull resolution‎ (1,052 × 1,100 pixels, file size: 344 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Caesium dodecarborate is an inorganic compound with the formula Cs2B12H12. ...


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 deficient and pose a problem for conventional descriptions of covalent bonding that involves shared electron pairs. BH3 is a trigonal planar molecule (D3h molecular symmetry). Diborane has a hydrogen bridged structure, see the diborane article. The description of the bonding in the larger boranes formulated by Lipscomb involved: Electron deficiency occurs when a compound has too few valence electrons for the connections between atoms to be described as covalent bonds. ... Covalent redirects here. ... ǃǁɚɵ A generic trigonal planar molecule showing ideal bond angle. ... Molecular symmetry in chemistry describes symmetry in molecules and the classification of molecules in groups based on symmetry. ... Diborane is a colorless gas at room temperature with a repulsive, sweet odor. ... Diborane is a colorless gas at room temperature with a repulsive, sweet odor. ...

  • 3 center 2 electron 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 BH2 groups.
Lipscombs methodology has largely been superseded by a molecular orbital approach although it still affords insights. The results of this have been summarised in a simple but powerful rule, PSEPT, 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 Stones tensor harmonic treatment. A recent developments is four-center two-electron bond. A three-center two-electron bond is an electron deficient chemical bond where three atoms share two electrons. ... The polyhedral skeletal electron pair theory or Wades rules in chemistry are electron counting rules that apply to electron-poor cages such as boron hydrides which consist of many boron atoms linked together. ... A 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 (2c-2e bonding). ...


Chemistry of boranes

Properties and reactivity trends

Boranes are all colourless and diamagnetic. They are reactive compounds and some are pyrophoric. The majority are highly poisonous and require special handling precautions. Levitating pyrolytic carbon Diamagnetism is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field. ... Plutonium pyrophoricity can cause it to look like a glowing ember under certain conditions. ...

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) and decaborane(14) 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.

Pentaborane is a chemical compound considered in the 1950s as a good prospect for a rocket or jet fuel by the U.S. armed services, a so-called exotic fuel. Its chemical structure is that of five atoms of boron compounded with nine atoms of hydrogen (B5H9). ... Decaborane (B10H14) is a stable, crystalline borane. ...

Synthesis and general reactivity

Borane BH3
This is an important intermediate in the pyrolosis of diborane to produce higher boranes.
Diborane
Diborane is made industrially by the reduction of BF3, and is the starting point for preparing the higher boranes.It has been studied extensively.
Higher boranes
Typical reactions of boranes are
    • electrophilic substitution
    • cleavage by Lewis bases
    • deprotonation by strong bases
    • cluster building reactions with borohydrides
    • reaction of a nido-borane with an alkyne to give a carborane cluster
Boranes can act as ligands in coordination compounds. Hapticities 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 salt to produce Fe(η2-B6H10)(CO)4.

Boranes can react to form hetero-boranes e.g.carboranes or metalloboranes (clusters that contain boron and metal atoms). Diborane is a colorless gas at room temperature with a repulsive, sweet odor. ... Diborane is a colorless gas at room temperature with a repulsive, sweet odor. ... Boron trifluoride is the chemical compound with the formula BF3. ... In chemistry, an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. ... A Lewis base is any molecule or ion that can form a new covalent bond by donating a pair of electrons. ... In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. ... // η-bonding The Greek letter η (eta) is used to denote the number of atoms of a ligand that bind to a metal center. ... ... 3D model of carborane acid, colors: Hydrogen - white, Chlorine - yellow, Boron - green, Carbon - black A carborane is a cluster composed of boron and carbon atoms. ... In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. ...


History

The development of the chemistry of boranes posed two challenges to chemists. Firstly new laboratory techniques had to be developed to handle these very reactive compounds and secondly, the structures of the compounds challenged the accepted theories of chemical bonding.
The German chemist Alfred Stock first characterized the series of boron-hydrogen compounds. His group developed the glass vacuum line and techniques for handling the compounds. Unfortunately exposure to mercury (used in mercury diffusion pumps and float valves) caused Stock to develop mercury poisoning which he documented in the first scientific papers on the subject. The chemical bonding of the borane clusters was investigated by Lipscomb and his co-workers. Lipscomb was awarded the Nobel prize in Chemistry in 1976 for this work. PSEPT, (Wades rules) can be used to predict the structures of boranes.
Interest in boranes increased during World War Two due to the potential of uranium borohydride for enrichment of the uranium isotopes. In the US, a team led by Schlesinger 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 reagents for organic synthesis, most of which were developed by his student Herbert C. Brown. Borane-based reagents are now widely used in organic synthesis. For example, sodium borohydride is the standard reagent for converting aldehydes and ketones to alcohols. Brown was awarded the Nobel prize in Chemistry in 1979 for this work.[2] In the 1950s and early sixties, the US and USSR invested in boron hydrides as high-energy fuels (ethylboranes, for example) for very fast aircraft such as the XB-70 Valkyrie. The development of advanced surface-to-air missiles made the fast aircraft redundant, and the fuel programs were terminated, although triethylborane (TEB) was later used to light the engines of the SR-71 Blackbird high-speed plane.[3] This is a list of famous chemists: (alphabetical order) Contents: Top - 0–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Emil Abderhalden, (1877–1950), Swiss chemist Richard Abegg, (1869–1910), German... Alfred Stock (July 16, 1876 - August 12, 1946) was a German inorganic chemist. ... Alfred Stock (July 16, 1876 - August 12, 1946) was a German inorganic chemist. ... In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. ... For other uses, see William Lipscomb (disambiguation). ... For other uses, see William Lipscomb (disambiguation). ... The Nobel Prize (Swedish: ), as designated in Alfred Nobels will in 1895, is awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace. ... The polyhedral skeletal electron pair theory or Wades rules in chemistry are electron counting rules that apply to electron-poor cages such as boron hydrides which consist of many boron atoms linked together. ... Uranium borohydride U(BH4)4 is a volatile uranium compound with boron. ... Hermann Irving Schlesinger (October 11, 1882 - October 3, 1960) was an American inorganic chemist, working in boron chemistry. ... A reagent is a material used to start a {[chemical reaction]}. For example hydrochloric acid is the chemical reagent that would cause calcium carbonate to release carbon dioxide. ... Organic synthesis is the construction of organic molecules via chemical processes. ... Herbert Charles Brown (May 22, 1912 – December 19, 2004) was a chemist who won the Nobel Prize in Chemistry in 1979 (along with Georg Wittig) for his work with organoboranes. ... Sodium borohydride, also known as sodium tetrahydroborate, has the chemical formula NaBH4. ... An aldehyde. ... Ketone group A ketone (pronounced as key tone) is either the functional group characterized by a carbonyl group (O=C) linked to two other carbon atoms or a chemical compound that contains this functional group. ... This article does not cite any references or sources. ... The Nobel Prize (Swedish: ), as designated in Alfred Nobels will in 1895, is awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace. ... The North American XB-70 Valkyrie was conceived for the Strategic Air Command in the 1950s as a high-altitude bomber that could fly three times the speed of sound (Mach 3). ... See Organoboranes TEB - Triethylborane was used to ignite the JP-7 fuel of the Pratt & Whitney J-58 ramjet engines powering the Lockheed SR-71 Blackbird spy plane. ... SR-71 redirects here. ...


General references

  1. Fox M.A., Wade K. Pure Appl. Chem. (2003),75,9, 1315
  2. Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
  3. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.

F. Albert Cotton is the W.T. Doherty-Welch Foundation Chair and Distinguished Professor of Chemistry at Texas A&M University. ... Sir Geoffrey Wilkinson was an English chemist He was born 14 July 1921 in the village of Springside, near Todmorden in Yorkshire. ...

Footnotes

  1. ^ Peymann T., Knobler C.B.,Khan S.I., Hawthorne M.F. Angew. Chemie Intnl Ed.(2001) 40,9,1664
  2. ^ Brown, H. C. “Organic Syntheses via Boranes” John Wiley & Sons, Inc. New York: 1975. ISBN 0-471-11280-1.
  3. ^ http://incolor.inebraska.com/hwolfe/history/sr71.pdf

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