Editing Boron hydride clusters (section)

==Bonding in boranes==
Boranes are nonclassically–bonded compounds, that is, there are not enough electrons to form 2-centre, 2-electron bonds between all pairs of adjacent atoms in the molecule. A description of the bonding in the larger boranes was formulated by [[William Lipscomb]]. It 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 {{chem2|BH2}})
Lipscomb's methodology has largely been superseded by a [[molecular orbital]] approach. This allows the concept of multi-centre bonding to be extended. For example, in the icosahedral ion {{chem2|[B12H12](2-)}}, the totally symmetric (A<sub>g</sub> symmetry) molecular orbital is equally distributed among all 12 boron atoms. [[Polyhedral skeletal electron pair theory|Wade's rules]] provide a powerful method that can be used to rationalize the structures in terms of the number of atoms and the connectivity between them.

===Multicluster boranes===
[[File:GAKFEF.png|thumb|Structure of the conjuncto boron hydride cluster {{chem2|[B19H22]−}}.<ref>{{cite journal |doi=10.1021/ic901976y|title=An Experimental Solution to the "Missing Hydrogens" Question Surrounding the Macropolyhedral 19-Vertex Boron Hydride Monoanion &#91;B19H22&#93;−, a Simplification of its Synthesis, and its Use as an Intermediate in the First Example of syn-B18H22 to anti-B18H22 Isomer Conversion|year=2010|last1=Londesborough|first1=Michael G. S.|last2=Bould|first2=Jonathan|last3=Baše|first3=Tomáš|last4=Hnyk|first4=Drahomír|last5=Bakardjiev|first5=Mario|last6=Holub|first6=Josef|last7=Císařová|first7=Ivana|last8=Kennedy|first8=John D.|journal=Inorganic Chemistry|volume=49|issue=9|pages=4092–4098|pmid=20349936}}</ref>]]
Although relatively rare, several multi-cluster boranes have been characterized. For example, reaction of a borane cluster with {{chem2|B2H6}} (as a source of {{chem2|BH3}}) can lead to the formation of a ''conjuncto''-borane species in which borane cluster sub-units are joined by the sharing of boron atoms.<ref>{{Greenwood&Earnshaw2nd}} p. 162</ref>
:{{chem2|B6H10 + "BH3" → B7H11 + H2}}
:{{chem2|B7H11 + B6H10 → B13H19 + H2}}
Other ''conjuncto''-boranes, where the sub-units are joined by a B-B bond, can be made by ultra violet irradiation of ''nido''-boranes. Some B-B coupled ''conjuncto''-boranes can be produced using {{chem2|PtBr2}} as catalyst.<ref>{{cite journal |last1=Sneddon |first1=L.G. |title=Transition metal promoted reactions of polyhedral boranes and carboranes |journal=Pure and Applied Chemistry |date=2009 |volume=59 |issue=7 |pages=837–846 |doi=10.1351/pac198759070837|s2cid=55817512 |doi-access=free}}</ref>

Analogous to Wade's Rules, electron counting scheme has been developed to predict or rationalize multicluster boranes.
{| class="wikitable"
|+Multi-cluster descriptors<ref>{{cite journal |last1=Bould |first1=Jonathan |last2=Clegg |first2=William |last3=Teat |first3=Simon J. |last4=Barton |first4=Lawrence |last5=Rath |first5=Nigam P. |last6=Thornton-Pett |first6=Mark |last7=Kennedy |first7=John D. |title=An approach to megalo-boranes. Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds. Crystal structure determinations by conventional and synchrotron methods |journal=Inorganica Chimica Acta |date=1999 |volume=289 |issue=1–2 |pages=95–124 |doi=10.1016/S0020-1693(99)00071-7}}</ref>
! Prefix !! Meaning !! Example
|-
| ''klado''- || branched clusters ||
|-
| ''conjuncto''- || conjoined clusters ||
|-
| ''megalo''-|| multiple conjoined clusters ||
|}
===Lewis acid/base behavior===
Some function as electron donors owing to the relative basic character of the {{chem2|B\sH_{terminal}|}} groups. Boranes can function as [[ligand]]s in [[coordination compound]]s.<ref name="BoranesAsLigands">{{Greenwood&Earnshaw2nd|page=177, "The concept of boranes as ligands"}}</ref> [[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-''{{chem2|B6H10}} can replace ethene in [[Zeise's salt]] to produce {{chem2|''trans''\-Pt(η^{2}\-B6H10)Cl2}}.<ref name="BoranesAsLigands"/>

They can also act as [[Lewis acid]]s, with concomitant opening of the cluster. An example involving [[trimethylphosphine]]:
:{{chem2|B5H9 + 2 P(CH3)3 → B5H9*2P(CH3)3}}

===Brønsted acid/base behavior===
Some higher boranes, especially those with bridging hydrogen atoms, can be deprotonated with a strong base. An example:
:{{chem2|B5H9 + NaH → Na[B5H8] + H2}}
Acidity increases with the size of the borane, with {{chem2|B10H14}} having a [[pKa|p''K''<sub>a</sub>]] value of 2.7:<ref>{{Greenwood&Earnshaw2nd}} p. 171</ref>
:{{chem2|[[Pentaborane(9)|B5H9]] < [[Hexaborane(10)|B6H10]] < [[Decaborane(14)|B10H14]] < [[Hexadecaborane(20)|B16H20]] < [[Octadecaborane|B18H22]]}}
In general, bridging hydrogen protons tend to be lost before terminal ones.<ref>{{Greenwood&Earnshaw2nd}}</ref>

===Aufbau reactions===
[[File:KIWJOP.png|thumb|180 px|Structure of {{chem2|[(CH3)4N+]2[Fe(C2B9H11)2]+}}, showing only one {{chem2|Me4N+}}.<ref>{{cite journal|author1=Kang, H. C. |author2=Lee, S. S. |author3=Knobler, C. B. |author4=Hawthorne, M. F. |title=Syntheses of Charge-Compensated Dicarbollide Ligand Precursors and Their Use in the Preparation of Novel Metallacarboranes|journal=Inorganic Chemistry|year=1991|volume=30|issue=9|pages=2024–2031|doi=10.1021/ic00009a015}}</ref>]]

For the boron hydride chemist, one of the most important reactions is the building up process by which smaller boron hydride clusters add borane to give larger clusters. This approach also applies to the synthesis of [[metallaborane]]s,

===Hydroboration===
Reminiscent of the behavior of diborane and its adducts, higher boranes participate in hydroboration. When boron hydrides add an [[alkyne]], the carbon becomes incorporated into the cluster, producing [[carborane]]s, e.g. {{chem2|C2B10H12}}.<ref>{{cite journal | author = Jemmis, E. D. | author-link = Eluvathingal Devassy Jemmis | title = Overlap control and stability of polyhedral molecules. Closo-Carboranes | journal = Journal of the American Chemical Society | year = 1982 | volume = 104 | issue = 25 | pages = 7017–7020 | doi = 10.1021/ja00389a021}}</ref>