Borane
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[[Image:Borane-3D-balls.png|thumb|right|200px|[[Ball-and-stick model]] of borane, BH<sub>3</sub>, which is highly reactive.]]
[[Image:Diborane-3D-balls-A.png|thumb|right|200px|BH<sub>3</sub> dimerises to diborane, B<sub>2</sub>H<sub>6</sub>.]]
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 B<sub>x</sub>H<sub>y</sub>. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The parent member BH<sub>3</sub> is called borane, but it is known only in the gaseous state, and [[dimer]]ises to form [[diborane]], B<sub>2</sub>H<sub>6</sub>. The larger boranes all consist of boron [[cluster chemistry|clusters]] that are [[polyhedra|polyhedral]], some of which exist as [[isomer]]s. For example, isomers of B<sub>20</sub>H<sub>26</sub> are based on the fusion of two 10-atom clusters.
The most important boranes are [[diborane]] B<sub>2</sub>H<sub>6</sub>, [[pentaborane]] B<sub>5</sub>H<sub>9</sub>, and [[decaborane]] B<sub>10</sub>H<sub>14</sub>.<br/>
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 [[carborane]]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:-
{| class="wikitable"
|-
! Type
! formula
! notes
|-
| ''closo<nowiki>−</nowiki>''
| B<sub>n</sub>H<sub>n</sub><sup>2<nowiki>−</nowiki></sup>
| No neutral B<sub>n</sub>H<sub>n+2</sub> boranes are known
|-
| ''nido<nowiki>−</nowiki>''
| B<sub>n</sub>H<sub>n+4</sub>
|
|-
| ''arachno<nowiki>−</nowiki>''
| B<sub>n</sub>H<sub>n+6</sub>
|
|-
|''hypho<nowiki>−</nowiki>''
|B<sub>n</sub>H<sub>n+8</sub>
|only adducts established
|}
There also exists a series of substituted neutral ''hypercloso-''boranes that have the theoretical formulae B<sub>n</sub>H<sub>n</sub>. Examples include B<sub>12</sub>(OCH2Ph)<sub>12</sub>, which is a stable derivative of ''hypercloso-''B<sub>12</sub>H<sub>12</sub><ref>Peymann T., Knobler C.B.,Khan S.I., Hawthorne M.F. Angew. Chemie Intnl Ed.(2001) 40,9,1664</ref>.
===Naming conventions===
The [[International Union of Pure and Applied Chemistry nomenclature|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:- <br />
*B<sub>5</sub>H<sub>9</sub> pentaborane(9)
*B<sub>6</sub>H<sub>12</sub> hexaborane(12)
<br/>
The [[International Union of Pure and Applied Chemistry nomenclature|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:-
*B<sub>5</sub>H<sub>8</sub><sup><nowiki>−</nowiki></sup> octahydropentaborate(1<nowiki>−</nowiki>)
<br/>Optionally '''''closo<nowiki>−</nowiki>''''' '''''nido<nowiki>−</nowiki>''''' etc (see above) can be added:-
*B<sub>5</sub>H<sub>9</sub>, ''nido<nowiki>−</nowiki>''pentaborane(9)
*B<sub>4</sub>H<sub>10</sub>, ''arachno<nowiki>−</nowiki>''tetraborane(10)
*B<sub>6</sub>H<sub>6</sub><sup>2<nowiki>−</nowiki></sup>, hexahydro''<nowiki>−</nowiki>closo<nowiki>−</nowiki>''hexaborate(2<nowiki>−</nowiki>)
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 [[deltahedron|deltahedra]] that are found in borane chemistry are (using the names favoured by most chemists):--
{| class="wikitable"
|-
! 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 [[deltahedron|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 <sup>11</sup>B [[NMR spectroscopy|NMR]] spectra.
<gallery>
Image:Diborane-3D-balls.png|<center>[[Diborane|Diborane, B<sub>2</sub>H<sub>6</sub>]]</center>
Image:Pentaborane-3D-balls.png|<center>[[Pentaborane|Pentaborane-[9], B<sub>5</sub>H<sub>9</sub>]]</center>
Image:Decaborane-3D-balls.png|<center>[[Decaborane|Decaborane-[14], B<sub>10</sub>H<sub>14</sub>]]</center>
Image:Dodecaborane-3D-balls.png|<center>[[Caesium dodecaborate|B<sub>12</sub>H<sub>12</sub><sup>2-</sup>]]</center>
</gallery>
B<sub>6</sub>H<sub>10</sub> 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<sub>8</sub>H<sub>12</sub>, which would be expected to have a ''nido-'' geometry (based on B<sub>9</sub>H<sub>9</sub><sup>2<nowiki>−</nowiki></sup> missing 1 vertex), but is similar in geometry to B<sub>8</sub>H<sub>14</sub>, which is based on B<sub>10</sub>H<sub>10</sub><sup>2<nowiki>−</nowiki></sup>.<br />
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., B<sub>8</sub>Cl<sub>8</sub> is a slightly distorted dodecahedron
*'''''closo-''''' (from the Greek for "cage") a closed complete cluster, e.g., icosahedral B<sub>12</sub>H<sub>12</sub><sup>2<nowiki>−</nowiki></sup>
*'''''nido-''''' (from the Latin for "nest") B occupies n vertices of an n+1 deltahedron, e.g., B<sub>5</sub>H<sub>9</sub> an octahedron missing 1 vertex
*'''''arachno-''''' (from the Greek for "spiders web") B occupies n vertices of an n+2 deltahedron e.g. B<sub>4</sub>H<sub>10</sub> an octahedron missing 2 vertices
*'''''hypho-''''' (from the Greek for "net") B occupies n vertices of an n+3 deltahedron possibly B<sub>8</sub>H<sub>16</sub> has this structure, an octahedron missing 3 vertices
*'''''conjuncto-''''' 2 or more of the above are fused together
===Bonding in boranes===
Boranes are [[electron deficiency|electron-deficient]] and pose a problem for conventional descriptions of [[covalent bond|covalent]] bonding that involves shared electron pairs. BH<sub>3</sub> is a [[trigonal planar]] molecule (D<sub>3h</sub> [[molecular symmetry]]). [[Diborane]] has a hydrogen-bridged structure, see the [[diborane]] article.
The description of the bonding in the larger boranes formulated by [[William Lipscomb]] involved:
* [[three-center two-electron bond|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 BH<sub>2</sub>)
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<sub>2</sub> groups.<br/>
Lipscomb's 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, [[polyhedral skeletal electron pair theory|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 Stone's tensor surface harmonic treatment of cluster bonding. A recent development is [[four-center two-electron bond]].
===Chemistry of boranes===
====Properties and reactivity trends====
Boranes are all colourless and [[diamagnetism|diamagnetic]]. They are reactive compounds and some are [[pyrophoricity|pyrophoric]]. The majority are highly poisonous and require special handling precautions.
;''closo<nowiki>−</nowiki>''
:There is no known neutral closo borane. Salts of the closo anions, B<sub>n</sub>H<sub>n</sub><sup>2<nowiki>−</nowiki></sup> are stable in neutral aqueous solution, and their stabilities increase with size. The salt K<sub>2</sub>B<sub>12</sub>H<sub>12</sub> is stable up to 700<sup>o</sup>.
;''nido<nowiki>−</nowiki>''
:[[pentaborane|Pentaborane(9)]] and [[decaborane|decaborane(14)]] are the most stable ''nido''<nowiki>−</nowiki>boranes, in contrast to ''nido''<nowiki>−</nowiki>B<sub>8</sub>H<sub>12</sub> that decomposes above -35<sup>o</sup>.
;''arachno<nowiki>−</nowiki>''
:Generally these are more reactive than ''nido''<nowiki>−</nowiki>boranes and again larger compounds tend to be more stable.
====Synthesis and general reactivity====
;Borane BH<sub>3</sub>
:This is an important intermediate in the pyrolosis of diborane to produce higher boranes.
;[[Diborane]] B<sub>2</sub>H<sub>6</sub> and higher boranes
:[[Diborane]] is made industrially by the reduction of [[boron trifluoride|BF<sub>3</sub>]], and is the starting point for preparing the higher boranes.It has been studied extensively.
;General reactivity
:Typical reactions of boranes are
**[[electrophile|electrophilic]] substitution
**[[nucleophilic]] substitution by [[Lewis base]]s
**[[deprotonation]] by strong bases
**cluster building reactions with [[borohydride]]s
**reaction of a ''nido-''borane with an [[alkyne]] to give a carborane [[cluster chemistry|cluster]]
:Boranes can act as [[ligand]]s in [[coordination compound]]s. [[Hapticity|Hapticities]] of η<sup>1</sup> to η<sup>6</sup> have been found, with electron donation involving bridging H atoms or donation from B-B bonds. For example, ''nido-''B<sub>6</sub>H<sub>10</sub> can replace ethene in [[Zeise's salt]] to produce Fe(η<sup>2</sup>-B<sub>6</sub>H<sub>10</sub>)(CO)<sub>4</sub>.<br/>
Boranes can react to form hetero-boranes, e.g., [[carborane]]s or metalloboranes ([[cluster chemistry|clusters]] 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.<br />
The [[List of famous Germany people|German]] [[List of chemists|chemist]] [[Alfred Stock]]
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 [[Alfred Stock|Stock]] to develop mercury poisoning, which he documented in the first scientific papers on the subject. The chemical bonding of the borane [[cluster chemistry|clusters]] was investigated by [[William Nunn Lipscomb|Lipscomb]] and his co-workers. [[William Nunn Lipscomb|Lipscomb]] was awarded the [[Nobel prize]] in Chemistry in 1976 for this work. [[Polyhedral skeletal electron pair theory|PSEPT]], (Wades rules) can be used to predict the structures of boranes.<br />
Interest in boranes increased during World War II due to the potential of [[uranium borohydride]] for enrichment of the uranium isotopes. In the US, a team led by [[Hermann Irving Schlesinger|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 [[reagent]]s 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 [[aldehyde]]s and [[ketone]]s to [[alcohol]]s. Brown was awarded the [[Nobel prize]] in Chemistry in 1979 for this work.<ref>Brown, H. C. “Organic Syntheses via Boranes” John Wiley & Sons, Inc. New York: 1975. ISBN 0-471-11280-1.</ref>
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 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 ignite the engines of the [[SR-71 Blackbird]].<ref>http://incolor.inebraska.com/hwolfe/history/sr71.pdf</ref>
== General references ==
#Fox M.A., Wade K. Pure Appl. Chem. (2003),75,9, 1315
#{{Greenwood&Earnshaw}}
#{{Cotton&Wilkinson6th}}
== Footnotes ==
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[[Category:Boranes]]
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