Metal halides

A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride.

For example, with sodium hydroxide:[1] 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:[1][3] The silver and thallium(I) cations have a great affinity for halide anions in solution, and the metal halide quantitatively precipitates from aqueous solution.

Due to a smaller crystal field splitting energy, the halide complexes of the first transition series are all high spin when possible.

Alfred Werner studied hexamminecobalt(III) chloride, and was the first to propose the correct structures of coordination complexes.

The two chloride ligands are easily displaced, allowing the platinum center to bind to two guanine units, thus damaging DNA.

Due to the presence of filled pπ orbitals, halide ligands on transition metals are able to reinforce π-backbonding onto a π-acid.

[9] The volatility of the tetrachloride and tetraiodide complexes of Ti(IV) is exploited in the purification of titanium by the Kroll and van Arkel–de Boer processes, respectively.

Ferric and aluminium chlorides are catalysts for the Friedel-Crafts reaction, but due to their low cost, they are often added in stoichiometric quantities.

Mentioned above, the halide compounds can be made anhydrous by heat, vacuum, or treatment with thionyl chloride.

For example, sodium cyclopentadienide reacts with ferrous chloride to yield ferrocene:[11] While inorganic compounds used for catalysis may be prepared and isolated, they may at times be generated in situ by addition of the metal halide and the desired ligand.