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.[1]
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.[1]
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.[1]
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.[1]
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