Disodium enneaborate

Disodium enneaborate is the traditional name for a salt of sodium, boron, oxygen, and hydrogen, with elemental formula Na₂B₉H₂₂O₂₀ or Na₂B₉O₉·11H₂O. It is the sodium borate with the highest boron/sodium ratio.

Disodium enneaborate

Identifiers
3D model (JSmol)	Interactive image
InChI InChI=1S/B8H4O15.BH3O3.2Na.2H2O/c9-1-14-2(10)17-6(16-1)19-7-18-5(13)22-8(23-7)20-3(11)15-4(12)21-8;2-1(3)4;;;;/h9-12H;2-4H;;;2*1H2/q-2;;2*+1;; Key: FUFQAFYJPUNFLR-UHFFFAOYSA-N
SMILES [Na+].[Na+].[O]B1O[B-]2(OB(O)OB(O)O2)OB(O1)O[B-]3OB(O)OB(O)O3.OB(O)O.O.O
Properties
Chemical formula	B9H22Na2O20
Molar mass	485.43 g·mol−1
Related compounds
Related compounds	Borax Sodium pentaborate Disodium octaborate Trisodium borate Sodium metaborate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Structure

The correct formula has since been determined to be (Na⁺)₂[B₈O₁₁(OH)₄]²⁻·B(OH)₃·2H₂O. The anion is a linear polymer with repeating unit [−B₈O₁₁(OH)₄−]²⁻. Sodium cations, water molecules, and undissociated boric acid molecules B(OH)₃ lie between the chains, held by numerous hydrogen bonds.

The compound crystallizes in the monoclinic crystal system with space group P2₁/n. The cell parameters are a = 1021.3 pm, b = 1294.0 pm, c = 1245.7 pm, β = 93.070°, V = 1.6440 nm³, and Z = 2. The sodium cations occur in groups of four with interatomic distances of 378.30 pm and 379.32 pm.

Reactions

Upon heating, disodium enneaborate initially becomes amorphous and then crystallizes as anhydrous disodium octaborate α-Na₂B₈O₁₃ along with amorphous B₂O₃. Notably, the former contains octaborate fundamental building blocks that are topologically equivalent to those in the enneaborate.