Potassium ferrioxalate

Potassium ferrioxalate
Names
IUPAC name
Potassium iron(III) oxalate
Other names
potassium ferrioxalate
potassium trisoxalatoferrate(III)
Identifiers
ECHA InfoCard 100.035.398
Properties
K
3[Fe(C
2O
4)
3] (anhydrous)
K
3[Fe( C
2O
4)3]·3H
2O (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) the trihydrate loses 3H2O at 113 °C[1]
Structure
octahedral
0 D
Hazards
Main hazards Corrosive. Eye, respiratory and skin irritant.
R-phrases (outdated) R20, R21, R22, R34, R36/37/38
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).
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Infobox references

Potassium ferrioxalate, also known as potassium trisoxalatoferrate(III), is a chemical compound with the formula K
3[Fe(C
2O
4)
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, K
3[Fe(C
2O
4)
3]·3H
2O, are emerald green in color. In solution, the salt dissociates to give the ferrioxalate anion, [Fe(C
2O
4)
3]3−, which appears fluorescent green in color. Potassium ferrioxalate is often used in chemical actinometry, i.e. the measure of light flux.

Preparation

Crystals of potassium ferrioxalate trihydrate.

The complex can be synthesized by the reaction between iron(III) sulfate, barium oxalate and potassium oxalate:[2]

Fe
2(SO
4)
3 + 3 BaC
2O
4 + 3 K
2C
2O
4 → 2 K
3[Fe(C
2O
4)
3] + 3 BaSO
4

The reactants are combined in water, the solid precipitate of BaSO
4 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.[3]

FeCl
3·6H
2O + 3 K
2C
2O
4·H
2O → K
3[Fe(C
2O
4)
3]·3H
2O + 3 KCl + 6H
2O

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[4] 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).[5]

Reactions

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 CO
2. 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(C
2O
4)
3]3− + hν → 2 [Fe(C
2O
4)
2]2− + 2 CO
2 + C
2O2−
4[6]

Thermal decomposition

The trihydrate loses the three water molecules at the same time when heated at 113 °C.[1]

At 296 °C, the anhydrous salt decomposes into the iron(II) complex potassium ferrooxalate, potassium oxalate, and carbon dioxide:[1]

2 K
3[Fe(C
2O
4)
3] → 2 K
2[Fe(C
2O
4)
2] + K
2C
2O
4 + 2 CO
2

See also

A number of other iron oxalates are known

References

  1. 1 2 3 J. Ladriere (1992): "Mössbauer study on the thermal decomposition of potassium tris (oxalato) ferrate(III) trihydrate and bis (oxalato) ferrate(II) dihydrate". Hyperfine Interactions, volume 70, issue 1, pages 1095–1098. doi:10.1007/BF02397520
  2. ↑ Bailar, John C.; Jones, Eldon M. (1939). "Trioxalato Salts (Trioxalatoaluminiate, -ferriate, -chromiate, and -cobaltiate)". Inorg. Synth. 1: 35–38. doi:10.1002/9780470132326.ch13.
  3. ↑ Performed by Benjamin J. Abram, OSU lab manual, Dr. Richard Nafshun
  4. ↑ Junk, Peter C. (2005). "Supramolecular interactions in the X-ray crystal structure of potassium tris(oxalato)ferrate(III) trihydrate". J. Coord. Chem. 58 (4): 355–361. doi:10.1080/00958970512331334250.
  5. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
  6. ↑ Pozdnyakov, Ivan P.; Kel, Oksana V.; Plyusnin, Victor F.; Grivin, Vyacheslav P.; Bazhin, Nikolai M. (2008). "New Insight into Photochemistry of Ferrioxalate". J. Phys. Chem. A. 112 (36): 8316–8322. doi:10.1021/jp8040583.
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