Metal halides

Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. Covalently bonded metal halides may be discrete molecules, such as uranium hexafluoride, or they may form polymeric structures, such as palladium chloride.

The halogens can all react with metals to form metal halides according to the following equation:

where M is the metal, X is the halogen, and MX_n is the metal halide.

Sodium chloride crystal structure

Sodium chloride crystal structure

Discrete UF₆ molecules

Discrete UF₆ molecules

Infinite chains of one form of palladium chloride

Infinite chains of one form of palladium chloride

Preparation

In principle, most metal halides may be prepared by direct combination of the elements. For example, tin tetrachloride:

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: