Properties of Nickel Chloride

Nickel Chloride (or just nickel chloride), is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. It is very rarely found in nature as mineral nickelbischofite. A dihydrate is also known. In general nickel(II) chloride, in various forms, is the most important source of nickel for chemical synthesis. Nickel salts are carcinogenic. They are also deliquescent, absorbing moisture from the air to form a solution.
Syntheses
Probably the largest scale production of nickel chloride involves the extraction with hydrochloric acid of nickel matte and residues obtained from roasting refining nickel-containing ores.
NiCl2·6H2O is rarely prepared in the laboratory because it is inexpensive and has a long shelf-life. The hydrate can be converted to the anhydrous form upon heating in thionyl chloride or by heating under a stream of HCl gas. Simply heating the hydrates does not afford the anhydrous dichloride.
NiCl2·6H2O + 6 SOCl2 → NiCl2 + 6 SO2 + 12 HCl
The dehydration is accompanied by a color change from green to yellow.
Properties
Nickel Chloride adopts the CdCl2 structure. In this motif, each Ni2+ center is coordinated to six Cl- centers, and each chloride is bonded to three Ni(II) centers. In NiCl2 the Ni-Cl bonds have “ionic character”. Yellow NiBr2 and black NiI2 adopt similar structures, but with a different packing of the halides, adopting the CdI2 motif.
In contrast, NiCl2·6H2O consists of separated trans-[NiCl2(H2O)4] molecules linked more weakly to adjacent water molecules. Note that only four of the six water molecules in the formula are bound to the nickel, and the remaining two are water of crystallisation. Cobalt(II) chloride hexahydrate has a similar structure.
Many nickel(II) compounds are paramagnetic, due to the presence of two unpaired electrons on each metal center. Square planar nickel complexes are, however, diamagnetic.
Nickel(II) chloride solutions are acidic, with a pH of around 4 due to the hydrolysis of the Ni2+ ion.
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