Chemical Properties of Boron

Boron is the sole non-metal of group 13 and is comparatively unreactive at ordinary temperature, though it becomes reactive at high temperatures. A great deal of its chemistry is electron-deficient covalent bonding, a consequence of its small size, high ionisation energy, and an electronegativity close enough to carbon's and hydrogen's own that boron forms genuinely covalent bonds with both.

Formation of metal borides. Every metal except the alkali metals reacts with boron to give a boride, of general formula MₓBᵧ (x up to 11, y up to 66 or more). Borides can be made by direct combination of the metal with boron at high temperature, e.g. Cr + nB →(1500 K)→ CrBₙ, or by reducing boron trichloride with a metal in the presence of hydrogen gas, e.g. 2BCl₃ + 2W + H₂ →(1500 K)→ 2WB + 2Cl₂ + 2HCl.

Formation of hydrides. Boron does not react with hydrogen directly. Its hydrides -- called boranes -- are instead made indirectly; the simplest, diborane (B₂H₆), is obtained by treating gaseous boron trifluoride with sodium hydride around 450 K, the product being trapped immediately to stop it pyrolysing further: 2BF₃ + 6NaH →(450 K)→ B₂H₆ + 6NaF. Larger boranes can then be built up from diborane (Section 2.2.6).

Formation of boron trihalides. At high temperature, boron combines directly with any halogen to give the corresponding trihalide: 2B + 3X₂ →(Δ)→ 2BX₃.

Formation of boron nitride. Boron burns in dinitrogen at high temperature to form boron nitride: 2B + N₂ →(Δ)→ 2BN.

Formation of oxides. Heating boron in oxygen around 900 K gives its oxide: 4B + 3O₂ →(900 K)→ 2B₂O₃.

Reaction with acids and alkali. Ordinary halo acids do not react with boron at all. However, boron does react with strongly oxidising acids -- sulphuric acid and nitric acid -- to give boric acid: 2B + 3H₂SO₄ → 2H₃BO₃ + 3SO₂ and B + 3HNO₃ → H₃BO₃ + 3NO₂. Boron also reacts with fused sodium hydroxide to give sodium borate: 2B + 6NaOH → 2Na₃BO₃ + 3H₂. …