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| Identifiers | |
|---|---|
3D model (JSmol) | |
| ChemSpider | |
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| Properties | |
| C6FeO123− | |
| Molar mass | 319.901 g·mol−1 |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Ferrioxalate ortrisoxalatoferrate(III) is atrivalentanion with formula[Fe(C2O4)3]3−. It is atransition metal complex consisting of aniron atom in the +3 oxidation state and threebidentateoxalate ionsC2O2−4 anions acting asligands.
The ferrioxalate anion gives a lime green color to salts, and in solution it isfluorescent. The anion is sensitive to light and higher-energyelectromagnetic radiation, which causes the decomposition of one oxalate tocarbon dioxide (CO2) and reduction of the iron(III) atom to iron(II). This property is exploited foractinometry.
The most common and most-studied salt ispotassium ferrioxalate, but thesodium,ammonium, andlithium salts have also received some attention.
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In the absence of light or other radiation, the ferrioxalate complex is quite stable. The potassium and sodium salts and their solutions can be heated to near 100 °C for hours without significant decomposition.
The complex is held together bydative covalent bonds, due to the oxygen atoms in the oxalate anions (the "ligands") donating a lone pair to the p and d orbitals of the iron atom (the "center" of the complex). The center has three electrons in its d orbitals, leaving 13 empty places in the remaining d and p orbitals. Twelve of these are filled by electrons from the ligands.
The iron center in the ferrioxalate anion has a distortedoctahedral geometry. The ferrioxalate complex has D3molecular symmetry, within which the six Fe–O bond distances all close to 2.0Å[1] which indicates that the Fe(III) ishigh spin; as the low spin complex would displayJahn–Teller distortions. The ammonium and mixed sodium-potassium salts areisomorphous, as are related complexes with Al3+, Cr3+, and V3+.
The ferrioxalate complex displayshelical 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 symbolΛ (lambda). Its mirror image with the right-handed screw axis is given the Greek symbolΔ (delta).[2]
In solution, the ferrioxalate complex undergoesphotoreduction. In this process, the complex absorbs aphoton of light and subsequently decomposes to formFe(C
2O
4)2−
2 andCO
2. The iron centre isreduced (gains an electron) from the +3 to the +2oxidation state, while an oxalate ion is oxidised tocarbon dioxide:
This reaction provides an efficient chemical method forphotometry andactinometry, the measurement of light and higher-energy electromagnetic radiation. Potassium ferrioxalate is over 1000 times more sensitive thanuranyl oxalate, the compound previously used for these purposes.[3][4]While the complex itself is insensitive toneutrons, thelithium salt can be used to measure them. A lithium-6 nucleus can absorb a neutron and emitalpha particle4He2+ and atriton3H+ with high energies, which presumably decompose the nearby ferrioxalate.[5]