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