6.5 Group 13 halides

Trihalides, mx ₃

As shown in [link] all the combinations of Group 13 element (M) and halogen (X) exist for the trihalides (MX ₃ ), except thallium(III) iodide. It should be noted that while there is a compound with the general formula TlI ₃ , it is actually a thallium(I) compound of I ₃ ^- .

The trihalides of boron are all monomers with a coordination number of 3 ( [link] ), as evidence from their low melting points ( [link] ). In contrast, the fluorides and chlorides of the heavier Group 13 elements (except GaCl ₃ ) are generally ionic or have a high ionic character, with a coordination number of 6 ( [link] , [link] and [link] ). The bromides and iodides (except InBr ₃ ) are generally dimeric with a coordination number of 4 ( [link] ) and have molecular structures involving halide bridging ligands ( [link] and [link] ). AlCl ₃ is unusual in that in the solid state it has an ionic structure, but it is readily sublimed, and in the vapor phase (and liquid phase) it has a dimeric structure ( [link] ).

Synthesis

Boron trifluoride (BF ₃ ) is manufactured commercially by the reaction of boron oxides with hydrogen fluoride, [link] . The HF is produced in-situ from sulfuric acid and fluorite (CaF ₂ ). On smaller scales, BF ₃ is prepared by the thermal decomposition of diazonium salts, [link] .

Boron trichloride is also made from boron oxide, but in the presence of carbon, [link] .

Many of the trihalides are readily prepared by either the direct reaction of the metal with the appropriate halogen, [link] - [link] , or the acid, [link] and [link] . Thallium tribromide can be prepared in CH ₃ CN by treating a solution of the monobromide with bromine gas, [link] .

Reactivity

The reaction chemistry of the Group 13 trihalides tends to fall into two categories:

• Lewis acid-base complex formation.
• Hydrolysis.

There are, however, a number of reactions involving halide exchange reactions. Aluminum tribromide reacts with carbon tetrachloride at 100 °C to form carbon tetrabromide, [link] , and with phosgene yields carbonyl bromide and aluminum chlorobromide, [link] .