Boranes

Decaborane-[14], B₁₀H₁₄is the most widely studied boron hydride cluster.

In chemistry, boranes comprise a large group of the group 13 hydride compounds with the generic formula of B_xH_y. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The class is named after the parent chemical called "borane" itself, chemical formula BH₃. This compound is only known to exist as a transient intermediate since it dimerises to form diborane, B₂H₆. The larger boranes all consist of boron clusters that are polyhedral. In addition to the charge-neutral boranes, a large number of anionic boron hydrides are known. The most important boranes are diborane B₂H₆ and two of its pyrolysis products, pentaborane B₅H₉ and decaborane B₁₀H₁₄. 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, but the only commercial applications involve derivatives of borane.

Generic formula of boranes

The four series of single-cluster boranes have the following general formulae, where "n" is the number of boron atoms:

There also exists a series of substituted neutral hypercloso-boranes that have the theoretical formulae B_nH_n. Examples include B₁₂(OCH₂Ph)₁₂, a derivative of the unstable 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:

• 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⁻ octahydridopentaborate(1−)

Optionally closo− nido− etc. can be added:

• B₅H₉, nido−pentaborane(9)
• B₄H₁₀, arachno−tetraborane(10)
• B₆H₆²⁻, hexahydrido−closo−hexaborate(2−)

Understandably many of the compounds have abbreviated common names.

Cluster types

The geometries of boron clusters are related. Generally they are described as (i) deltahedra, polyhedra with triangular faces or (ii) those same deltahedra with one or more vertices missing. Common deltahedra that are found in boron-hydride clusters are shown below.

Vertices Deltahedron Image 5 Trigonal bipyramid 6 Octahedron 7 Pentagonal bipyramid 8 Dodecahedron (snub disphenoid)

Vertices Deltahedron Image 9 Tricapped trigonal prism (triaugmented triangular prism) 10 Bicapped square antiprism (gyroelongated square bipyramid) 11 Edge-contracted icosahedron 12 Icosahedron

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.

• 

  Borane, BH₃, a highly reactive and rarely observed borane.

• 

  Diborane, B₂H₆

• 

  Tetraborane, B₄H₁₀

• 

  Pentaborane-[9], B₅H₉

• 

  Decaborane-[14], B₁₀H₁₄

• 

  B₁₂H₁₂²⁻

B₆H₁₀ 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 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⁻.

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 deltahedron cluster, with 2n+2 framework electrons e.g., icosahedral B₁₂H₁₂²⁻
• nido- (from the Latin for "nest") B occupies n vertices of an n+1 deltahedron with 2n+4 framework electrons, e.g., B₅H₉ an octahedron missing 1 vertex
• arachno- (from the Greek for "spiders web") B occupies n vertices of an n+3 deltahedron with 2n+6 framework electrons e.g. B₄H₁₀ an octahedron missing 2 vertices
• hypho- (from the Greek for "net") B occupies n vertices of an n+2 deltahedron, e.g., B₈H₁₆ which is trigonal prismatic
• klado- (from the Greek for "branch")
• conjuncto- 2 or more of the above are fused together, e.g., the edge or two vertex fused B₁₉H₂₂¹⁻, face or three vertex fused B₂₁H₁₈¹⁻, and four vertex fused B₂₀H₁₆

B₁₈H₂₂

iso-B₁₈H₂₂

Isomeric conjuncto clusters

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 (D_3h 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:

• 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.

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, 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.

Synthesis

Diborane is made industrially by the reduction of BF₃, and is the starting point for preparing the higher boranes. It has been studied extensively. The higher (larger) boranes are all derived from diborane. Heating this compound generates the reactive monomer BH₃, which combines with diborane to produce higher boranes.

Reactivity of boranes

Boranes are all colourless and diamagnetic. They are reactive compounds and some are pyrophoric. The majority are highly poisonous and require special handling precautions.

closo−

There is no known neutral closo borane. Salts of the closo anions, B_nH_n²⁻ are stable in neutral aqueous solution, and their stabilities increase with size. The salt K₂B₁₂H₁₂ is stable up to 700 °C.

nido−

Pentaborane(9) and decaborane(14) are the most stable nido−boranes, in contrast to nido−B₈H₁₂ that decomposes above -35^o.

arachno−

Generally these are more reactive than nido−boranes and again larger compounds tend to be more stable.

Typical reactions of boranes are

• electrophilic substitution
• nucleophilic substitution by Lewis bases
• deprotonation by strong bases
• cluster building reactions by pyrolysis and by condensation with borohydrides, which serves as a Lewis base.
• reaction of a nido-borane with an alkyne to give a carborane cluster

Boranes can function 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).

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 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 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 II 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, Schlesinger'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 1960s, the US and USSR investigated boron hydrides as high energy density jet fuel ("zip fuel") (ethylboranes, for example) for high speed aircraft,^[3] such as the XB-70 Valkyrie.^[4] 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.^[5]

General references

1. Fox, Mark A.; Wade, Ken (2003). "Evolving patterns in boron cluster chemistry". Pure Appl. Chem. 75 (9): 1315–1323. doi:10.1351/pac200375091315. 
2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9. 
3. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 

Footnotes

Binary compounds of hydrogen
Alkali metal hydrides	LiH NaH KH RbH CsH	Lithium hydride, LiH ionic metal hydride Beryllium hydride Left (gas phase): BeH2 covalent metal hydride Right: (BeH2)n (solid phase) polymeric metal hydride Borane and diborane Left: BH3 (special conditions), covalent metalloid hydride Right: B2H6 (standard conditions), dimeric metalloid hydride Methane, CH4 covalent nonmetal hydride Ammonia, NH3 covalent nonmetal hydride Water, H2O covalent nonmetal hydride Hydrogen fluoride, HF covalent nonmetal hydride
Alkaline earth hydrides	Monohydrides BeH MgH CaH SrH BaH BeH2 MgH2 CaH2 SrH2 BaH2
Group 13 hydrides	Boranes BH3 B2H6 B2H4 B2H2 Alanes AlH3 Al2H6 Gallanes GaH3 Ga2H6 Indiganes InH3 In2H6 Thallanes TlH3 Tl2H6 B2H2 B2H4 B4H10 B5H9 B5H11 B6H10 B6H12 B10H14 B18H22
Group 14 hydrides	Linear alkanes CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 more... Linear alkenes C2H4 C3H6 C4H8 C5H10 C6H12 C7H14 C8H16 C9H18 C10H20 more... Linear alkynes C2H2 C3H4 C4H6 C5H8 C6H10 C7H12 C8H14 C9H16 C10H18 more... Silanes SiH4 Si2H6 Si3H8 Si4H10 Si5H12 Si6H14 Si7H16 Si8H18 Si9H20 Si10H22 more... Silenes Si2H4 Silynes Si2H2 Germanes GeH4 Ge2H6 Ge3H8 Ge4H10 Ge5H12 Stannanes SnH4 Sn2H6 Plumbanes PbH4 CH CH2 CH3 C2H Cycloalkanes Cycloalkenes Annulenes Many more
Pnictogen hydrides	Azanes NH3 N2H4 N3H5 N4H6 N5H7 N6H8 N7H9 N8H10 N9H11 N10H12 more... Azenes N2H2 N3H3 N4H4 Phosphanes PH3 P2H4 P3H5 P4H6 P5H7 P6H8 P7H9 P8H10 P9H11 P10H12 more... Phosphenes P2H2 P3H3 P4H4 Arsanes AsH3 As2H4 Stibanes SbH3 Bismuthanes BiH3 HN3 NH radical
Hydrogen chalcogenides	Polyoxidanes H2O H2O2 H2O3 H2O4 H2O5 H2O6 H2O7 H2O8 H2O9 H2O10 more... Polysulfanes H2S H2S2 H2S3 H2S4 H2S5 H2S6 H2S7 H2S8 H2S9 H2S10 more... Selanes H2Se H2Se2 Tellanes H2Te H2Te2 Polanes PoH2 HO HO2 HO3 H2O+–O– H2S=S (HS)2S+–S– HS HDO D2O T2O
Hydrogen halides	HF HCl HBr HI HAt
Transition metal hydrides	ScH2 YH2 YH3 TiH2 ZrH2 HfH2 VH VH2 NbH NbH2 TaH CrH CrH2 CrHx NiH PdHx (x < 1) FeH FeH2 CuH ZnH2 CdH2 HgH2
Lanthanide hydrides	LaH2 LaH3 CeH2 CeH3 PrH2 PrH3 NdH2 NdH3 SmH2 SmH3 EuH2 GdH2 GdH3 TbH2 TbH3 DyH2 DyH3 HoH2 HoH3 ErH2 ErH3 TmH2 TmH3 YbH2 YbH2.5 LuH2 LuH3
Actinide hydrides	AcH2 ThH2 Th4H15 PaH3 UH3 NpH2 NpH3 PuH2 PuH3 AmH2 AmH3 CmH2