Nickel(II) chloride

Overview
Nickel(II) chloride (or just nickel chloride), is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. 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.

Production and 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.

Structure and properties
NiCl2 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.

Coordination chemistry
Most of the reactions ascribed to “nickel chloride” involve the hexahydrate, although specialized reactions require the anhydrous form. For example, NiCl2 reacts with dimethoxyethane (dme) to form the molecular complex NiCl2(dme)2. This complex reacts with sodium cyclopentadienylide to give the sandwich compound nickelocene.

Reactions starting from NiCl2·6H2O can be used to form a vast array of nickel coordination complexes because the H2O ligands are rapidly displaced by ammonia, amines, thioethers, thiolates, and organophosphines. In some derivative, the chloride remains within the coordination sphere, whereas chloride is displaced with highly basic ligands chloride. Illustrative complexes include:
 * violet, paramagnetic, octahedral [Ni(NH3)6]Cl2
 * orange, diamagnetic, square planar NiCl2(dppe)
 * colorless, diamagnetic, square planar [Ni(CN)4]2-
 * blue, paramagnetic, tetrahedral [NiCl4]2-

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. For example, NiCl2(PPh3)2, containing four-coordinate Ni(II), exists in solution as a mixture of both the diamagnetic square planar and the paramagnetic tetrahedral isomers. Square planar complexes of nickel can often form five-coordinate adducts.

NiCl2 is the precursor to Ni(acac)2 (with a trimeric structure), which is a key precursor to Ni(1,5-cyclooctadiene)2, an important reagent in organonickel chemistry.

Applications in organic synthesis
NiCl2 and its hydrate are occasionally useful in organic synthesis.
 * As a mild Lewis acid, e.g. for the regioselective isomerization of dienols:
 * In combination with CrCl2 for the coupling of an aldehyde and a vinylic iodide to give allylic alcohols.
 * For selective reductions in the presence of LiAlH4, e.g. for the conversion of alkenes to alkanes.
 * As a precursor to “nickel boride”, prepared in situ from NiCl2 and NaBH4. This reagent behaves like Raney Nickel, comprising an efficient system for hydrogenation of unsaturated carbonyl compounds.
 * 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)3 → ArP(O)(OEt)2 + EtI

Other uses
Nickel chloride solutions are used for electroplating. It is also used in batteries.