Ferrioxalate

{{Short description|Ion}} {{Chembox |ImageFile = Ferrioxalate.svg |ImageSize = 150px |Section1={{Chembox Identifiers |CASNo = 15321-61-6 |ChemSpiderID = 146351 |PubChem = 167279 |StdInChI=1S/3C2H2O4.Fe/c3*3-1(4)2(5)6;/h3*(H,3,4)(H,5,6);/q;;;+3/p-6 |StdInChIKey = FYJLUEWYCIBBGT-UHFFFAOYSA-H |SMILES = C(=O)(C(=O)[O-])[O-].C(=O)(C(=O)[O-])[O-].C(=O)(C(=O)[O-])[O-].[Fe+3] }} |Section2={{Chembox Properties |Fe=1|C=6|O=12 |Formula_Charge = 3- }} }}

'''Ferrioxalate''' or '''trisoxalatoferrate(III)''' is a trivalent anion with formula {{chem2|[Fe(C2O4)3](3-)}}. It is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions {{chem2|C2O4(2-)}} anions acting as ligands.

The ferrioxalate anion gives a lime green color to salts, and in solution it is fluorescent. The anion is sensitive to light and higher-energy electromagnetic radiation, which causes the decomposition of one oxalate to carbon dioxide ({{chem2|CO2}}) and reduction of the iron(III) atom to iron(II). This property is exploited for actinometry.

The most common and most-studied salt is potassium ferrioxalate, but the sodium, ammonium, and lithium salts have also received some attention.

==Properties== [[File:Potassium trioxalatoferrate(III) large crystals.jpg|thumb|left|Potassium ferrioxalate crystals.]] ===Stability=== 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.

===Molecular structure=== The complex is held together by dative 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 distorted octahedral geometry. The ferrioxalate complex has D₃ molecular symmetry, within which the six Fe–O bond distances all close to 2.0 Å<ref>{{cite journal|title=Supramolecular interactions in the X-ray crystal structure of potassium tris(oxalato)ferrate(III) trihydrate|last=Junk |first=Peter C. |journal=J. Coord. Chem. |date=2005 |volume=58|issue=4 |pages=355–361 |doi=10.1080/00958970512331334250|s2cid=216142329 }}</ref> 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⁺.

===Chirality=== The ferrioxalate complex displays helical chirality as it can form two non-superposable 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).<ref>{{Greenwood&Earnshaw}}</ref> 300px|frameless|center

==Reactions==

===Photoreduction=== In solution, the ferrioxalate complex undergoes photoreduction. In this process, the complex absorbs a photon of light and subsequently decomposes to form {{chem|Fe(C|2|O|4|)|2|2−}} and {{chem|C|O|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 [{{chem|Fe|(|C|2|O|4|)|3}}]³⁻ + ''hν'' → 2 [{{chem|Fe|(|C|2|O|4|)|2}}]²⁻ + 2 {{chem|C|O|2}} + {{chem|C|2|O|4|2−}} This reaction provides an efficient chemical method for photometry and actinometry, the measurement of light and higher-energy electromagnetic radiation. Potassium ferrioxalate is over 1000 times more sensitive than uranyl oxalate, the compound previously used for these purposes.<ref>{{cite journal|title=A new sensitive chemical actinometer. II. Potassium ferrioxalate as a standard chemical actinometer|author1=Hatchard, C. G. |author2=Parker, C. A. |journal=Proceedings of the Royal Society of London|year=1956|volume=235|issue=1203|pages=518–36|doi=10.1098/rspa.1956.0102|bibcode=1956RSPSA.235..518H|s2cid=98652159}}</ref><ref>{{cite journal|title=New Insight into Photochemistry of Ferrioxalate |first1=Ivan P. |last1=Pozdnyakov |first2=Oksana V. |last2=Kel |first3=Victor F. |last3=Plyusnin |first4=Vyacheslav P. |last4=Grivin |first5=Nikolai M. |last5=Bazhin |journal=J. Phys. Chem. A |date=2008 |volume=112 |issue=36 |pages=8316–8322 |doi=10.1021/jp8040583|pmid=18707071 |bibcode=2008JPCA..112.8316P }}</ref> <!-- NEEDS SOURCES If a solution containing both green ferrioxalate ions and colourless free oxalate ions is exposed to strong light, such as direct sunlight, the light allows the Iron-III to oxidize one of the oxalate ligands to carbon dioxide and gives the orange-brown ferrooxalate complex ion which is coordinated around an Iron-II centre, however, when placed in the dark the Iron-II is re-oxidized to Iron-III by the oxygen in the atmosphere and the green ferrioxalate complex ion re-forms. The orange-brown Iron-II complex starts to appear after around ten minutes exposure and after the passage of a few hours in direct sunlight more than half of the green Iron-III complex had been reduced. The re-oxidation in the dark is equally slow and observable under ambient electric lighting. If this process is allowed to repeat over many months, such as leaving a container outside where it is exposed to the sun each day, eventually almost all of the oxalate ions present are oxidized to carbonate and the iron remains as Ferric Hydroxide, Fe(OH)₃. This indicates that when exposed to the environment, particularly if that environment is damp the ferrioxalate ion is quite unstable and gradually decomposes via the above redox processes into much more stable and common compounds.--> While the complex itself is insensitive to neutrons, the lithium salt can be used to measure them. A lithium-6 nucleus can absorb a neutron and emit alpha particle {{chem2|^{4}He(2+)}} and a triton {{chem2|^{3}H(+)}} with high energies, which presumably decompose the nearby ferrioxalate.<ref name=akashi>Junko Akashi, Yoshio Uchida, Tomoko Kojima, Motomi Katada, and Hirotoshi Sano (1984): "Mössbauer Spectroscopic Studies of the Effects of the 6Li(n, α)T Reaction in Lithium Tris(oxalato)ferrate(III)". ''Bulletin of the Chemical Society of Japan'', volume 57, issue 4, pages 1076-1078. {{doi|10.1246/bcsj.57.1076}}</ref>

==See also== *Sodium ferrioxalate *Iron(III) oxalate

==References == {{reflist}}

Category:Anions Category:Iron complexes Category:Oxalato complexes Category:Light-sensitive chemicals