Ammonia borane

Ammonia borane

Ball and stick model of ammonia borane
Names
IUPAC name Ammoniotrihydroborate
Other names Borazane
Identifiers
CAS Number	13774-81-7 N
ChemSpider	371215 Y
Jmol 3D model	Interactive image
PubChem	419330
InChI InChI=1S/BH6N/c1-2/h1-2H3 N Key: PJYXJGDRFASJSB-UHFFFAOYSA-N N
SMILES [BH3][NH3]
Properties
Chemical formula	BNH 6
Molar mass	30.865 g mol−1
Appearance	Colorless crystals
Density	780 mg mL−1
Melting point	104 °C (219 °F; 377 K)
Structure
Crystal structure	I4mm, tetragonal
Coordination geometry	Tetragonal at B and N
Molecular shape	Tetrahydral at B and N
Dipole moment	5.2 D
Vapor pressure	{{{value}}}
Related compounds
Related compounds	Sodium borohydride Borazine Borane Ethane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Ammonia borane (or borazane) is the chemical compound with the formula H₃NBH₃. The colourless solid is the simplest molecular boron-nitrogen-hydride compound and has attracted attention as a source of hydrogen fuel.

Synthesis

Reaction of diborane with ammonia mainly gives the diammoniate salt [H₂B(NH₃)₂]⁺(BH₄)⁻. Ammonia borane is the main product when an adduct of borane is employed in place of diborane:

BH₃(THF) + NH₃ → BH₃NH₃ + THF

Properties and structure

The molecule adopts a structure like ethane, with which it is isoelectronic. The B-N distance is 1.58(2) Å. The B-H and N-H distances are 1.15 and 0.96 Å, respectively. Its similarity to ethane is tenuous since borane-ammonia is a solid and ethane is a gas: their melting points differing by 284 °C. This difference is consistent with the highly polar nature of ammonia borane. The H atoms attached to boron are hydridic and those attached to nitrogen are somewhat acidic.

Resonance structures of ammonia-borane

The structure of the solid indicates a close association of the NH and the BH centers. The closest H--H distance is 1.990 Å, which can be compared with the H-H bonding distance of 0.74 Å. This interaction is called a dihydrogen bond. The original crystallographic analysis of this compound reversed the assignments of B and N. The updated structure was arrived at with improved data using the technique of neutron diffraction that allowed the hydrogen atoms to be located with greater precision.

Part of the crystal structure of ammonia borane[2]

Uses

Ammonia borane has been suggested as a storage medium for hydrogen when the gas is used to fuel motor vehicles. It can be made to release hydrogen on heating, being polymerized first to (NH₂BH₂)_n, then to (NHBH)_n, which ultimately decomposes to boron nitride (BN) at temperatures above 1000^oC. It is more hydrogen-dense than liquid hydrogen and also able to exist at normal temperatures and pressures.

Ammonia borane finds some use in organic synthesis as an air-stable derivative of diborane.