Nickel(II) Chloride Hexahydrate
Properties
| State | Solid (crystalline) |
| Color | Emerald green |
| Solubility | Very soluble in water (2540 g/L at 20°C); soluble in ethanol |
| Melting Point | 140°C (loses water); anhydrous NiCl2 melts at 1001°C |
| Boiling Point | Loses water stepwise upon heating |
About Nickel(II) Chloride Hexahydrate
Nickel(II) chloride hexahydrate is the green-jewel form of nickel chloride — formula NiCl2·6H2O, molar mass 237.691 g/mol — and the green isn't from chloride at all. It's the [Ni(H2O)6]²⁺ hexaaquanickel(II) octahedron, where six water molecules ligate around the d8 nickel center and crystal-field splitting puts the absorption maximum near 720 nm in the red, leaving green light to pass through. Compare that with anhydrous NiCl2 (yellow) and the color shift on dehydration is one of the cleanest demonstrations of ligand-field effects you can run in an undergrad lab — heat a watch glass of the green hexahydrate and you'll watch it bleach to yellow within a few minutes. Industrially, NiCl2·6H2O lives in the Watts nickel-plating bath alongside NiSO4·6H2O and boric acid, where the chloride ion does two specific jobs: it boosts solution conductivity (cutting tank voltage) and it activates the nickel anodes so they dissolve cleanly instead of passivating. In synthesis, NiCl2·6H2O is the cheapest entry into nickel catalysis — Suzuki, Negishi, Kumada, and the modern photoredox/Ni dual-catalysis manifolds all start with a precursor like this, often reduced in situ. It's also the standard precursor for Raney nickel.
Where you'll encounter it
If you've ever weighed out a nickel catalyst for a cross-coupling and watched the balance pan turn faintly green where you spilled, that's NiCl2·6H2O picking up moisture from the air — the salt is hygroscopic and will deliquesce in a humid hood. In an electroplating shop, you can spot a Watts bath instantly: it's the bright kelly-green tank running at 50–60°C with nickel anode bags suspended from the bus bar, and the green darkens as nickel concentration climbs. In organic chem, undergraduate students often meet this salt as the 'nickel boride' precursor for selective alkyne semireduction — adding NaBH4 to NiCl2·6H2O in methanol generates a finely divided black catalyst (P-2 nickel) that hydrogenates triple bonds to cis-alkenes without overreducing.
Common Uses
- Nickel ion source in Watts electroplating baths for steel, brass, and zinc die-castings
- Precursor for Ni(0) and Ni(II) cross-coupling catalysts in Suzuki, Negishi, and Kumada reactions
- Starting material for Raney nickel and nickel boride (P-2) hydrogenation catalysts
- Ligand-field theory teaching demonstration via dehydration color change from green to yellow
- Precursor for nickel hydroxide cathode material in NiMH and nickel-cadmium batteries
- Chemical vapor deposition source for nickel thin films in semiconductor processing
- Mordant in nickel-complex acid dye formulations for wool and nylon fibers
Safety Information
GHS classifications: H301 (toxic if swallowed), H315 (skin irritation), H317 (skin sensitization), H331 (toxic if inhaled), H334 (respiratory sensitization), H341 (suspected mutagen), H350i (may cause cancer by inhalation), H360D (reproductive toxicity), H410 (very toxic to aquatic life with long-lasting effects). IARC Group 1 carcinogen as nickel compounds. OSHA PEL is 1 mg/m3 as Ni for soluble nickel compounds (8-hour TWA); ACGIH TLV is 0.1 mg/m3. Nickel sensitization affects roughly 17% of women and 3% of men, making it the most common cause of allergic contact dermatitis worldwide. Handle with nitrile gloves, lab coat, and dust mask; weigh in a hood. Aqueous waste must be collected as heavy-metal hazardous waste — never sink-disposed.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.