Potassium ferrioxalate

Potassium ferrioxalate

Lime green crystals of potassium ferrioxalate trihydrate
Potassium ferrioxalate
Names
IUPAC name Potassium iron(III) oxalate
Other names potassium ferrioxalate potassium trisoxalatoferrate(III)
Identifiers
CAS Number	5936-11-8 Y 5936-11-8 (trihydrate) N
Properties
Chemical formula	K3[Fe(C2O4)3] (anhydrous) K3[Fe(C2O4)3]·3H2O (trihydrate)
Molar mass	437.20 g/mol (anhydrous) 491.25 g/mol (trihydrate)
Appearance	emerald green hydrated crystals
Density	2.13 g/cm3
Melting point	230 °C (446 °F; 503 K)
Structure
Coordination geometry	octahedral
Dipole moment	0 D
Vapor pressure	{{{value}}}
Related compounds
Other anions	Sodium ferrioxalate
Related compounds	Iron(II) oxalate Iron(III) oxalate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Potassium ferrioxalate, also known as potassium trisoxalatoferrate(III), is a chemical compound with the formula K₃[Fe(C₂O₄)₃], where iron is in the +3 oxidation state. It is an octahedral transition metal complex in which three bidentate oxalate ions are bound to an iron center. Potassium acts as a counterion, balancing the −3 charge of the complex. Crystals of the trihydrated form of the complex, K₃[Fe(C₂O₄)₃]·3H₂O, are emerald green in color. In solution, the salt ionizes to give the ferrioxalate anion, [Fe(C₂O₄)₃]³⁻, which appears fluorescent green in color. Potassium ferrioxalate is often used in chemical actinometry, i.e. the measure of light flux.

Preparation

File:Potassium trisoxalatoiron(III) trihydrate.jpg

Crystals of potassium ferrioxalate trihydrate.

The complex can be synthesized by the reaction between iron(III) sulfate, barium oxalate and potassium oxalate:

Fe₂(SO₄)₃ + 3 BaC₂O₄ + 3 K₂C₂O₄ → 2 K₃[Fe(C₂O₄)₃] + 3 BaSO₄

The reactants are combined in water, the solid precipitate of BaSO₄ is removed, and the green trihydrate crystallizes from the cooled solution.

Another common synthesis is reacting aqueous iron(III) chloride hexahydrate and potassium oxalate monohydrate. Filter the solid using filter paper and a Büchner funnel and recrystallize the product to remove impurities.

FeCl₃·6H₂O + 3 K₂C₂O₄•H₂O → K₃[Fe(C₂O₄)₃]·3H₂O + 3 KCl + 6 H₂O

Structure

The ferrioxalate complex has D₃ molecular symmetry, within which the iron center is octahedral. The six Fe–O bond distances all close to 2.0 Å which indicates that the Fe(III) is high spin; as the low spin complex would display Jahn–Teller distortions. The ammonium and mixed sodium-potassium salts are isomorphous, as are related complexes with Al³⁺, Cr³⁺, and V³⁺.

The ferrioxalate complex displays helical chirality as it can form two non-superimposable geometries. In accordance with the IUPAC convention, the isomer with the left-handed screw axis is assigned the Greek symbol Λ (lambda). Its mirror image with the right-handed screw axis is given the Greek symbol Δ (delta).

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Photoreduction

In solution, the ferrioxalate complex undergoes photoreduction. In this process, the complex absorbs a photon of light and subsequently decomposes to form Fe(C
₂O
₄)²⁻
₂ and CO₂. The iron centre is reduced (gains an electron) from the +3 to the +2 oxidation state while an oxalate ion is oxidised to carbon dioxide:

2 [Fe(C₂O₄)₃]³⁻ + hv → 2 [Fe(C₂O₄)₂]²⁻ + 2 CO₂ + C
₂O²⁻
₄

See also

A number of other iron oxalates are known

• Iron(II) oxalate
• Iron(III) oxalate
• Sodium ferrioxalate