Some nickel chloride complexes exist as an equilibrium mixture of two geometries these examples are some of the most dramatic illustrations of structural isomerism for a given coordination number. In some derivatives, the chloride remains within the coordination sphere, whereas chloride is displaced with highly basic ligands. Reactions starting from NiCl2♶H2O can be used to form a variety of nickel coordination complexes because the H2O ligands are rapidly displaced by ammonia, amines, thioethers, thiolates, and organophosphines. From left to right, 2+, 2+, 2−, 2+ Most of the reactions ascribed to "nickel chloride" involve the hexahydrate, although specialized reactions require the anhydrous form. Color of various Ni(II) complexes in aqueous solution. Nickel(II) chloride solutions are acidic, with a pH of around 4 due to the hydrolysis of the Ni2+ ion. The trans sites on the octahedral centers occupied by aquo ligands. It consists of infinite chains of NiCl2, wherein both chloride centers are bridging ligands. The dihydrate NiCl2♲H2O adopts a structure intermediate between the hexahydrate and the anhydrous forms. The hexahydrate occurs in nature as the very rare mineral nickelbischofite. Cobalt(II) chloride hexahydrate has a similar structure. Only four of the six water molecules in the formula is bound to the nickel, and the remaining two are water of crystallization.
In contrast, NiCl2♶H2O consists of separated trans- molecules linked more weakly to adjacent water molecules. Yellow NiBr2 and black NiI2 adopt similar structures, but with a different packing of the halides, adopting the CdI2 motif. In NiCl2 the Ni-Cl bonds have "ionic character". In this motif, each Ni2+ center is coordinated to six Cl− centers, and each chloride is bonded to three Ni(II) centers. In case one needs a pure compound without presence of cobalt, nickel chloride can be obtained cautiously heating hexammine nickel chloride. The dehydration is accompanied by a color change from green to yellow. Simply heating the hydrates does not afford the anhydrous dichloride. The hydrates convert to the anhydrous form upon heating in thionyl chloride or by heating under a stream of HCl gas. Heating the hexahydrate in the range 66-133.☌ gives the yellowish dihydrate, NiCl2♲H2O. Nickel chloride is not usually prepared in the laboratory because it is inexpensive and has a long shelf-life. 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. Nickel salts have been shown to be carcinogenic to the lungs and nasal passages, in cases of long-term inhalation exposure. The nickel chlorides are deliquescent, absorbing moisture from the air to form a solution.
Nickel(II) chloride, in various forms, is the most important source of nickel for chemical synthesis. The anhydrous salt is yellow, but the more familiar hydrate NiCl2♶H2O is green. Nickel(II) chloride (or just nickel chloride), is the chemical compound NiCl2.
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