Iron(II) Chloride
Properties
| State | Solid (crystalline) |
| Color | Yellowish-green (anhydrous); blue-green (tetrahydrate) |
| Solubility | Soluble in water (648 g/L at 20°C) |
| Melting Point | 677°C |
| Boiling Point | 1023°C |
About Iron(II) Chloride
Iron(II) chloride is the textbook example of a high-spin d6 octahedral complex you'll never quite get to stay reduced. The anhydrous salt is a yellowish-green CdCl2-structured solid; the tetrahydrate FeCl2·4H2O that you actually buy is a pale blue-green crystal containing trans-[FeCl2(H2O)4] units. Dissolve it and you get pale green [Fe(H2O)6]^2+, the classic d6 octahedral aqua complex with a small but measurable crystal-field splitting that Inorganic 2 students invariably calculate from its absorption spectrum. The catch: Fe^2+(aq) sits about +0.77 V below Fe^3+(aq) on the standard reduction potential ladder, which means atmospheric O2 (E° = +1.23 V) oxidizes it readily. Open a freshly green FeCl2 solution to air and within hours it darkens through olive to rust as Fe^3+ hydrolyzes and Fe(OH)3 precipitates — which is why analytical procedures using Fe(II) (permanganate back-titration, the iron-thiocyanate method) demand fresh solutions or N2 sparging. Industrially FeCl2 is mainly a wastewater coagulant, where it both flocculates suspended solids and reduces hexavalent chromium. It's also a precursor to most other Fe compounds (Fe powder, Fe(III) salts via Cl2 oxidation, ferrocene via reduction) and a mild Lewis acid catalyst for some C-N coupling and radical chain reactions.
Where you'll encounter it
If you've ever titrated permanganate against Mohr's salt in lab and watched the pale green disappear with a single overshoot drop, or noticed an old FeCl2 stock bottle sitting at the back of the cabinet has gone rust-colored on top — both are the Fe^2+/Fe^3+ couple at work in real time. In a chrome-plating shop's effluent tank, FeCl2 dosed at slight stoichiometric excess reduces hexavalent chromium to Cr(III) in seconds, after which lime brings the pH up to ~9 and Cr(OH)3 floc settles cleanly. In an undergraduate inorganic prep lab, the 'make ferrocene from FeCl2 and sodium cyclopentadienide' procedure starts by stripping waters off the tetrahydrate under N2 in THF, because any residual moisture kills the NaCp before it can get to the iron. Both jobs hinge on keeping Fe^2+ from oxidizing first.
Common Uses
- Coagulant and flocculant in municipal and industrial wastewater treatment
- Reduction of hexavalent chromium Cr(VI) to Cr(III) in plating-shop effluent
- Precursor for ferrocene synthesis and other organoiron complexes
- Mild Lewis-acid catalyst for some Atom-Transfer Radical Cyclization (ATRC) reactions
- Etching solution component for printed circuit boards (alongside FeCl3)
- Mordant in textile dyeing for muted greens and blacks
Safety Information
GHS: H302 (harmful if swallowed), H315 (skin irritation), H319 (eye irritation), H290 (may be corrosive to metals). OSHA has no specific PEL; ACGIH TLV for soluble iron salts is 1 mg/m³ (as Fe). Solutions are acidic (~pH 3) due to Fe^2+ hydrolysis and will etch carbon steel and aluminum. Repeated ingestion at industrial doses causes hemochromatosis-like iron overload. Wear nitrile gloves and splash goggles. Store away from oxidizers (Cl2, HNO3, peroxides — Fe(II) is a reducing agent and will react vigorously).
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.