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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 - .

Selected physical properties of the Group 13 trihalides, MX 3 .
Element Mp (°C) Bp (°C)
BF 3 -126.8 -100.3
BCl 3 -107.3 12.6
BBr 3 -46.3 91.3
BI 3 49.9 210
AlF 3 1291 -
AlCl 3 192.4 (anhydrous), 0.0 (hexahydrate) 120 (hexahydrate)
AlBr 3 97.8 265
AlI 3 189.4 (anhydrous) 185 dec. (hexahydrate) 300 subl.
GaF 3 800 1000
GaCl 3 77.9 201
GaBr 3 121.5 278.8
GaI 3 212 345
InF 3 1172 -
InCl 3 586 800
InBr 3 220 -
InI 3 210 subl. -
TlF 3 300 dec. -
TlCl 3 40 dec. -
TlBr 3 40 dec. -

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] ).

The Group 13 element coordination numbers for the trihalides, MX 3 .
Element Fluoride Chloride Bromide Iodide
B 3 3 3 3
Al 6 6 (4) 4 4
Ga 6 4 4 4
In 6 6 6 4
Tl 6 6 4 -
Solid state structure of MF 3 (M = Al, Ga, In). Grey spheres represent the metal, while pale blue spheres represent fluorine.
Solid state structure of MCl 3 (M = Al, In, Tl) and MBr 3 (M = In). Grey spheres represent the metal, while green spheres represent the halogen.
Structure of GaCl 3 , MBr 3 (M = Al, Ga, Tl), and MI 3 (M = Al, Ga, In). Grey spheres represent the metal, while green spheres represent the halogen.
Selected bond lengths and angles for dimeric M 2 X 6 compounds. a X t and X b indicate terminal and bridging halides, respectfully.
Compound M-X t (Å) a M-X b (Å) a X t -M-X t (°) a X b -M-X b (°) a M-X-M (°)
Al 2 Br 6 2.21 2.33 115 93 87
In 2 I 6 2.64 2.84 125.1 93.7 86.3

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] .

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Source:  OpenStax, Chemistry of the main group elements. OpenStax CNX. Aug 20, 2010 Download for free at http://cnx.org/content/col11124/1.25
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