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Concept
Metal halides
Summary
Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride.
File:NaCl polyhedra.png|Sodium chloride crystal structure
File:Uranium-hexafluoride-unit-cell-3D-balls.png|Discrete UF6 molecules
File:Alpha-palladium(II)-chloride-xtal-3D-balls.png|Infinite chains of one form of palladium chloride
The halogens can all react with metals to form metal halides according to the following equation:
2M + nX2 → 2MXn
where M is the metal, X is the halogen, and MXn is the metal halide.
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 can sometimes 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 thallium(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.
Official source
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