Pentaborane(11)

Pentaborane(11) is inorganic compound with the chemical formula B₅H₁₁. It is an obscure boron hydride cluster, especially relative to the heavily studied pentaborane(9) (B₅H₉). With two more hydrogen atoms than nido-pentaborane(9), pentaborane(11) is classified as an arachno- cluster.^[1]

Pentaborane(11)

Identifiers
CAS Number	18433-84-6 Y
3D model (JSmol)	Interactive image
ChemSpider	34987102
ECHA InfoCard	100.038.447
EC Number	242-307-4
UNII	79Z6OH65UV Y
CompTox Dashboard (EPA)	DTXSID901317015
InChI InChI=1S/B5H11/c1-3-4-2-5(1,3)4/h5H,3-4H2,1-2H3 Key: OPSKXWPMAKCNOG-UHFFFAOYSA-N
SMILES [BH3]1[BH2]2[BH]13[BH2]2[BH3]3
Properties
Chemical formula	B5H11
Molar mass	65.14 g·mol−1
Melting point	−123 °C (−189 °F; 150 K)
Boiling point	63 °C (145 °F; 336 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Synthesis

Like many boron hydride clusters, pentaborane(11) was originally obtained from the pyrolysis of diborane. A more systematic synthesis entails treatment of [B₄H₉]⁻ with boron tribromide. The Lewis acid abstracts hydride to give unstable B₄H₈, the precursor to B₅H₁₁:^[2]

[B₄H₉]⁻ + BBr₃ → B₄H₈ + HBBr₃⁻

2 B₄H₈ → B₅H₁₁ + "B₃H₅"