Boron oxides, oxoacids and oxoanions

  The principal oxide of boron, B₂O₃, is obtained as a vitreous solid by dehydration of boric acid at red heat  or in a crystalline form by controlled dehydration.

The latter possesses a three-dimensional, covalent structure comprising planar BO₃ units (B^_O =138 pm) which share O atoms, but which are mutually twisted with respect to each other to give a rigid lattice. Under high pressure and at 803 K, a transition to a more dense form occurs, the change in density being 2.56 to 3.11 g cm^_3. This second polymorph contains tetrahedral BO₄ units, which are irregular because three O atoms are shared among three BO₄ units, while one atom connects two BO₄ units. Heating B2O3 with B at 1273K gives BO; its structure has not been established, but the fact that reaction with water yields (HO)₂BB(OH)₂ suggests it contains B^_B bonds. Trigonal planar and tetrahedral B exemplified in the polymorphs of B₂O₃ occur frequently in boron–oxygen chemistry.

   The commercial importance of B₂O₃ is in its use in the borosilicate glass industry. As a Lewis acid, B₂O₃ is a valuable catalyst; BPO₄ (formed by reacting B₂O₃ with P₄O₁₀) catalyses the hydration of alkenes and dehydration of amides to nitriles. The structure of BPO₄ can be considered in terms of SiO₂  in which alternate Si atoms have been replaced by B or P atoms.