Pentaborane

Pentaborane

Names
IUPAC name Pentaborane(9)
Other names Pentaborane, pentaboron nonahydride, stable pentaborane
Identifiers
CAS Number	19624-22-7 N
3D model (Jmol)	Interactive image
ChemSpider	24765156 Y
ECHA InfoCard	100.039.253
EC Number	243-194-4
InChI InChI=1S/B5H9/c6-1-5-2(6)8-4(5)9-3(5)7-1/h1-5H Y Key: USBVLEBZPMQADS-UHFFFAOYSA-N Y InChI=1/B5H9/c6-2-1-3(2,6)5(1,8-3)4(1,2,7-2)9-5/h1-5H Key: XPIBKKWNZBDJNI-UHFFFAOYAS
SMILES [H]1[BH]2[H][BH]3[BH]24[BH]1[H][BH]4[H]3
Properties
Chemical formula	B5H9
Molar mass	63.12 g/mol
Appearance	Colorless liquid
Odor	pungent, like sour milk
Density	0.618 g/mL
Melting point	−46.8 °C (−52.2 °F; 226.3 K)
Boiling point	60.1 °C (140.2 °F; 333.2 K)
Solubility in water	Reacts with water
Vapor pressure	171 mmHg (20°C)
Hazards
NFPA 704	4 4 2
Flash point	30 °C (86 °F; 303 K)
Explosive limits	0.42%-?
Lethal dose or concentration (LD, LC):
LD50 (median dose)	<50 mg/kg
LC50 (median concentration)	3 ppm (mouse, 4 hr) 6 ppm (rat, 4 hr) 3.4 ppm (mouse, 4 hr) 35 ppm (dog, 15 min) 244 ppm (monkey, 2 min) 67 ppm (rat, 5 min) 40 ppm (mouse, 5 min) 31 ppm (rat, 15 min) 19 ppm (mouse, 15 min) 15 ppm (rat, 30 min) 11 ppm (mouse, 30 min) 10 ppm (rat, 1 hr) 6 ppm (mouse, 1 hr)
US health exposure limits (NIOSH):
PEL (Permissible)	TWA 0.005 ppm (0.01 mg/m3)
REL (Recommended)	TWA 0.005 ppm (0.01 mg/m3) ST 0.015 ppm (0.03 mg/m3)
IDLH (Immediate danger)	1 ppm
Structure
Point group	C4v
Dipole moment	0 D
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N  (what is YN ?)
Infobox references

Pentaborane, also called pentaborane(9) to distinguish it from pentaborane(11) (B₅H₁₁), is an inorganic compound with the formula B₅H₉. It is one of the most common boron hydride clusters, although it is a highly reactive compound. Because of its high reactivity toward oxygen, it was once evaluated as rocket or jet fuel. Like many of the smaller boron hydrides, pentaborane is colourless, diamagnetic, and volatile.

Its structure is that of five atoms of boron arranged in a square pyramid. Each boron has a terminal hydride ligand and four hydrides span the edges of the base of the pyramid. It is classified as a nido cage.

It was first prepared by by pyrolysis of diborane at about 200 °C. An improved synthesis starts from salts of B₃H₈⁻, which is converted to the bromide B₃H₇Br⁻ using is HBr. Pyrolysis of this bromide gives pentaborane.

In the U.S., pentaborane was produced on a commercial scale by Callery Chemical Company.

Above 150 °C, it decomposes, producing hydrogen; when it occurs in a closed container, the consequent rise of pressure can be dangerous. It is much more stable in presence of water than diborane. It is soluble in hydrocarbons, benzene, and cyclohexane, and in greases including those used in lab equipment. In storage, it decomposes negligibly, yielding a small amount of hydrogen and solid residue.

The chemistry of pentaborane is extensive. Halogenation give the symmetrical derivatives B₅H₈X, which can be isomerised to place the halide on the base of the square pyramid. With strong bases such as alkyl lithium reagents, it can be deprotonated and the resulting lithium salts react with diverse electrophiles to give substituted derivatives. It is Lewis acidic, forming double adducts with two equivalents of trimethylphosphine. Pentaborane is used for the synthesis of other boron hydride clusters.