Ferrioxalate

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Ferrioxalate or trisoxalatoferrate(III) is a trivalent anion with formula [Fe(C
2
O
4
)
3
]3−
. It is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions C
2
O2−
4
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 CO
2
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[edit]

Stability[edit]

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[edit]

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[1] 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[edit]

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).[2]

2-isomers-of-ferrioxalate.svg

Reactions[edit]

Photoreduction[edit]

In solution, the ferrioxalate complex undergoes photoreduction. In this process, the complex absorbs a photon of light and subsequently decomposes to form Fe(C
2
O
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
2
O
4
)
3
]3− + → 2 [Fe(C
2
O
4
)
2
]2− + 2 CO
2
+ C
2
O2−
4

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.[3][4] 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 4
He2+
and a triton 3
H+
with high energies, which presumably decompose the nearby ferrioxalate.[5]

See also[edit]

References[edit]

  1. ^ 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.
  2. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  3. ^ Hatchard, C. G.; Parker, C. A. (1956). "A new sensitive chemical actinometer. II. Potassium ferrioxalate as a standard chemical actinometer". Proceedings of the Royal Society of London. 235: 518–36. doi:10.1098/rspa.1956.0102.CS1 maint: uses authors parameter (link)
  4. ^ 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.
  5. ^ 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