Nickel (II) chloride is the chemical compound NiCl 2 . The anhydrous salt is yellow, but the best known hydrate is NiCl 2 -6H 2 O is green. Very rarely they are found in nature as nickelbischofite mineral. There is also a dihydrate. In general, nickel (II) chloride is the most important source of nickel for chemical synthesis. Nickel salts are carcinogenic. Salts are also deliquescent, in that they absorb moisture from the air to form a solution.
Summary
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- 1 Production and synthesis
- 1 Structure and properties
- 2 Chemical coordination
- 2 Applications in organic synthesis
- 3 Other uses
- 4 Security
- 5 References
Production and synthesis
Large-scale production of nickel chloride involves extraction with nickel matte hydrochloric acid and residues obtained from roasting of nickel refining ore.
NiCl 2 -6H 2 O 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 by heating in thionyl chloride or by heating in a stream of HCl gas. Simple heating of the hydrates does not produce the anhydrous dichloride.
NiCl 2 -6H 2 O + 6 SOCl 2 → NiCl 2 + 6 SO 2 + 12 HCl
Dehydration is accompanied by a color change from green to yellow.
Structure and properties
NiCl 2 adopts the CdCl2 structure. In this arrangement, each Ni 2 + center is coordinated with six Cl- centers and each chlorine is connected with three Ni (II) centers.
In NiCl 2 the Ni-Cl junctions are “ionic in character”. In contrast, NiCl 2 -6H 2 O is made up of separate trans molecules – [NiCl 2 (H 2 O) 4] more loosely linked to adjacent water molecules. It is noteworthy that only four of the six water molecules in the formula are bound to nickel, and the remaining two are crystallizing water. Cobalt (II) chloride hexahydrate has a similar structure.
Many nickel (II) compounds are paramagnetic, due to the presence of two unpaired electrons in each metal center. However, flat nickel complexes are diamagnetic.
Nickel (II) chloride solutions are acidic, with a pH close to 4 due to hydrolysis of the Ni +2 ion.
Chemical coordination
Color of various concentrations of Ni (II) in aqueous solution.
From left to right, [Ni (NH 3 ) 6] 2 +, [Ni (ethylenediamine is) 3] 2 +, [NiCl 4 ] 2 -, [Ni (H 2 O) 6] 2 +
Most of the reactions attributed to “nickel chloride” involve hexahydrate, although specialized reactions require the anhydrous form.
Reactions from NiCl 2 -6H 2 O can be used to form a variety of nickel coordination complexes because H 2 O ligands are rapidly displaced by ammonia, amines, thioethers, thiolates, and phosphines. In some derivatives, the chloride remains within the coordination sphere, while the chloride is displaced by highly basic ligands. Some examples of complexes are:
Some nickel chloride complexes exist as an equilibrium mixture of two geometries; these examples are some of the most dramatic examples of structural isomerism for a given coordination number. For example, NiCl 2 (PPh 3 ) 2, which contains four Ni (II) coordinates, exists in solution as a mixture of the flat square diamagnetic and tetrahedral paramagnetic isomers. Flat nickel square complexes can often form five-coordinate adducts.
NiCl 2 is the precursor of the acetylacetonate complexes of Ni (acac) 2 (H 2 O) 2 and benzene- soluble (Ni (acac) 2) 3 , which is a precursor of Ni (1,5-cyclooctadiene) 2 , an important reagent in organic nickel chemistry.
In the presence of water scavengers, hydrated nickel chloride (II) reacts with dimethoxyethane (DME) to form the NiCl 2 (DME) 2 molecular complex . The DME ligands in this complex are labile. For example, this complex reacts with the cyclopentadienide sodium complex to give the nickellocene sandwich compound.
Organic synthesis applications
NiCl 2 and its hydrate are sometimes useful in organic synthesis.
- As a mild Lewis acid, for example, for regioselective isomerization of diols:
- In combination with CrCl 2for the coupling of an aldehyde and a vinyl iodide to give allyl alcohols .
- For selective reductions in the presence of LiAlH 4, for example, for the conversion of alkenes to alkanes .
- As a precursor to nickel boride , prepared in situ from NiCl 2. This reagent behaves like Raney nickel, which comprises an efficient system for the hydrogenation of unsaturated carbonyl compounds.
- As a precursor of finely divided Ni by reduction with Zn, for the reduction of aldehydes , alkenes, and aromatic nitro compounds. This reagent also promotes homo-coupling reactions, that is, 2RX → RR where R = vinyl, aryl.
- As a catalyst for making dialkyl phosphite arylphosphonates and aryl iodide, Ari:
Ari + P (OEt) 3 → ARP (O) (OEt) 2 + EtI
Other uses
Nickel chloride solutions are used for electroplating nickel in metal products.
Security
Nickel (II) chloride is irritating after ingestion, inhalation, contact with skin and eyes. Long-term exposure to nickel and its compounds has been shown to cause cancer.
