Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl<sub>2</sub>. The anhydrous salt is yellow, but the more familiar hydrate NiCl<sub>2</sub>·6H<sub>2</sub>O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for chemical synthesis. The nickel chlorides are deliquescent, absorbing moisture from the air to form a solution. Nickel salts have been shown to be carcinogenic to the lungs and nasal passages in cases of long-term inhalation exposure.
Production and syntheses
Large scale production and uses of nickel chloride are associated with the purification of nickel from its ores. It is generated upon extraction nickel matte and residues obtained from roasting refining nickel-containing ores using hydrochloric acid. Electrolysis of nickel chloride solutions are used in the production of nickel metal. Other significant routes to nickel chloride arise from processing of ore concentrates such as various reactions involving copper chlorides:
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Laboratory routes
Nickel chloride is not usually prepared in the laboratory because it is inexpensive and has a long shelf-life. The yellowish dihydrate, NiCl<sub>2</sub>·2H<sub>2</sub>O, is produced by heating the hexahydrate between 66 and 133 °C. The hydrates convert 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.
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The dehydration is accompanied by a color change from green to yellow.
Nickel chloride can also be obtained by cautiously heating hexaamminenickel chloride:
:<chem>\overset{hexammine\atop nickel~chloride}{[Ni(NH3)6]Cl2} ->[175-200^\circ\ce{C}] NiCl2{} + 6NH3</chem>
Structure of NiCl<sub>2</sub> and its hydrates
Structure of hydrated nickel chloride based on [[X-ray crystallography. Color code: red = O, green = Cl|thumb|left]]
NiCl<sub>2</sub> adopts the CdCl<sub>2</sub> structure. In this motif, each Ni<sup>2+</sup> center is coordinated to six Cl<sup>−</sup> centers, and each chloride is bonded to three Ni(II) centers. In NiCl<sub>2</sub> the Ni-Cl bonds have "ionic character". Yellow NiBr<sub>2</sub> and black NiI<sub>2</sub> adopt similar structures, but with a different packing of the halides, adopting the CdI<sub>2</sub> motif.
In contrast, NiCl<sub>2</sub>·6H<sub>2</sub>O consists of separated trans-[NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>] molecules linked more weakly to adjacent water molecules. Only four of the six water molecules in the formula is bound to the nickel, and the remaining two are water of crystallization, so the formula of nickel(II) chloride hexahydrate is [NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]·2H<sub>2</sub>O. A tetrahydrate NiCl<sub>2</sub>·4H<sub>2</sub>O is also known.
Reactions
Nickel(II) chloride solutions are acidic, with a pH of around 4 due to the hydrolysis of the Ni<sup>2+</sup> ion.
Coordination complexes
left|thumb|400px|Color of various Ni(II) complexes in aqueous solution. From left to right, , , ,
Most of the reactions ascribed to "nickel chloride" involve the hexahydrate, although specialized reactions require the anhydrous form.
Reactions starting from NiCl<sub>2</sub>·6H<sub>2</sub>O can be used to form a variety of nickel coordination complexes because the H<sub>2</sub>O ligands are rapidly displaced by ammonia, amines, thioethers, thiolates, and organophosphines. In some derivatives, the chloride remains within the coordination sphere, whereas chloride is displaced with highly basic ligands. Illustrative complexes include:
{| class="wikitable"
|-
! Complex
! Color
! Magnetism
! Geometry
|-
|
| blue/violet
| paramagnetic
| octahedral
|-
|
| violet
| paramagnetic
| octahedral
|-
|
| orange
| diamagnetic
| square planar
|-
|
| colorless
| diamagnetic
| square planar
|-
|
| blue
| paramagnetic
| tetrahedral
|}
NiCl<sub>2</sub> is the precursor to acetylacetonate complexes Ni(acac)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub> and the benzene-soluble (Ni(acac)<sub>2</sub>)<sub>3</sub>, which is a precursor to Ni(1,5-cyclooctadiene)<sub>2</sub>, an important reagent in organonickel chemistry.
In the presence of water scavengers, hydrated nickel(II) chloride reacts with dimethoxyethane (dme) to form the molecular complex NiCl<sub>2</sub>(dme)<sub>2</sub>.
- As a mild Lewis acid, e.g. for the regioselective isomerization of dienols:
::class=skin-invert-image|300px|General reaction scheme for the isomerisation of dienols
- In combination with CrCl<sub>2</sub> for the coupling of an aldehyde and a vinylic iodide to give allylic alcohols.
- For selective reductions in the presence of LiAlH<sub>4</sub>, e.g. for the conversion of alkenes to alkanes.
- As a precursor to Brown's P-1 and P-2 nickel boride catalyst through reaction with NaBH<sub>4</sub>.
- As a precursor to finely divided Ni by reduction with Zn, for the reduction of aldehydes, alkenes, and nitro aromatic compounds. This reagent also promotes homo-coupling reactions, that is 2RX → R-R where R = aryl, vinyl.
- As a catalyst for making dialkyl arylphosphonates from phosphites and aryl iodide, ArI:
::ArI + P(OEt)<sub>3</sub> → ArP(O)(OEt)<sub>2</sub> + EtI
NiCl<sub>2</sub>-dme (or NiCl<sub>2</sub>-glyme) is used due to its increased solubility in comparison to the hexahydrate.
center|344x344px|thumb|class=skin-invert-image|Application of NiCl<sub>2</sub> precatalyst.
Safety
Nickel(II) chloride is irritating upon ingestion, inhalation, skin contact, and eye contact. Prolonged inhalation exposure to nickel and its compounds has been linked to increased cancer risk to the lungs and nasal passages.