Preparation
In principle, most metal halides may be prepared by direct combination of the elements. For example, tin tetrachloride:
- Sn + 2 Cl2 → SnCl4
In practice, this type of reaction may be very exothermic, hence impractical as a preparative technique. Additionally, many transition metals can adopt multiple oxidation states, which complicates matters. As the halogens are strong oxidizers, direct combination of the elements usually leads to a highly oxidized metal halide. For example, ferric chloride can be prepared thus, but ferrous chloride cannot. Heating the higher halides may produce the lower halides; this occurs by thermal decomposition or by disproportionation. For example, gold(III) chloride to gold(I) chloride:
- AuCl3 → AuCl + Cl2 at 160°C
Metal halides are also prepared by the neutralization of a metal oxide, hydroxide, or carbonate with the appropriate halogen acid. For example, with sodium hydroxide:
- NaOH + HCl → NaCl + H2O
Water may be removed by heat, vacuum, or the presence of anhydrous hydrohalic acid. Anhydrous metal chlorides suitable for preparing other coordination compounds may be dehydrated by treatment with thionyl chloride:
- MCln·xH2O + x SOCl2 → MCln + x SO2 + 2x HCl
The silver and thalium(I) cations have a great affinity for halide anions in solution, and the metal halide quantitatively precipitates from aqueous solution. This reaction is so reliable that silver nitrate is used to test for the presence and quantity of halide anions. The reaction of silver cations with bromide anions:
- Ag+ (aq) + Br− (aq) → AgBr (s)
Read more about this topic: Metal Halides
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