Boron Trifluoride

Boron trifluoride, BF₃, is the boron trihalide of greatest chemical importance.

Preparation. Three routes: treating boron trioxide with calcium fluoride in the presence of conc. sulphuric acid, B₂O₃ + 3CaF₂ + 3H₂SO₄ →(Δ)→ 2BF₃ + 3CaSO₄ + 3H₂O; treating boron trioxide with carbon and fluorine, B₂O₃ + 3C + 3F₂ → 2BF₃ + 3CO; or, for laboratory-scale pure BF₃, thermally decomposing benzenediazonium tetrafluoroborate, C₆H₅N₂BF₄ →(Δ)→ BF₃ + C₆H₅F + N₂.

Properties. BF₃ has a planar (trigonal) geometry and, being electron-deficient at boron, readily accepts an electron pair to form coordinate covalent bonds, giving adducts with ammonia and water (BF₃ + NH₃ → F₃B←NH₃, BF₃ + H₂O → F₃B←OH₂) and, on hydrolysis, a family of complex fluoroborate anions of the type [BX₄]⁻. Full hydrolysis first gives boric acid, which is then converted to hydrofluoroboric acid: 4BF₃ + 12H₂O → 4H₃BO₃ + 12HF, then **3H₃BO₃ + 12HF → 3H₃O⁺ + 3[BF₄]⁻ + 9H …