Ferrioxalate or trisoxalatoferrate(III) is a

trivalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of an ...

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

with formula . It is a

transition metal complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many m ...

consisting of an

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

atom in the +3 oxidation state and three bidentate

oxalate Oxalate (IUPAC: ethanedioate) is an anion with the formula C2O42−. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl o ...

ions anions acting as

ligands In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electro ...

. The ferrioxalate anion gives a lime green color to salts, and in solution it is

fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

. The anion is sensitive to light and higher-energy

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...

, which causes the decomposition of one oxalate to

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

() and reduction of the iron(III) atom to iron(II). This property is exploited for

actinometry Actinometers are instruments used to measure the heating power of radiation. They are used in meteorology to measure solar radiation as pyranometers, pyrheliometers and net radiometers. An actinometer is a chemical system or physical device which ...

. The most common and most-studied salt is

potassium ferrioxalate Potassium ferrioxalate, also called potassium trisoxalatoferrate or potassium tris(oxalato)ferrate(III) is a chemical compound with the formula []. It often occurs as the trihydrate . Both are crystalline compounds, lime green in colour.A. Sarith ...

, but the sodium ferrioxalate, sodium, ammonium ferrioxalate, ammonium, and

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...

salts have also received some attention.

Properties

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 bond In coordination chemistry, a coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two-center, two-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal io ...

s, 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 In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The oc ...

. The ferrioxalate complex has D₃

molecular symmetry Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of these molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can be used to predict or explain m ...

, within which the six Fe–O bond distances all close to 2.0 Å which indicates that the Fe(III) is

high spin Spin states when describing transition metal coordination complexes refers to the potential spin configurations of the central metal's d electrons. For several oxidation states, metals can adopt high-spin and low-spin configurations. The ambiguity o ...

; as the low spin complex would display Jahn–Teller distortions. The ammonium and mixed sodium-potassium salts are

isomorphous In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word is ...

, as are related complexes with Al³⁺, Cr³⁺, and V³⁺.

Chirality

The ferrioxalate complex displays

helical chirality Helical may refer to: * Helix, the mathematical concept for the shape * Helical engine, a proposed spacecraft propulsion drive * Helical spring, a coilspring * Helical plc, a British property company, once a maker of steel bar stock * Helicoil A t ...

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-isomers-of-ferrioxalate

Reactions

Photoreduction

In solution, the ferrioxalate complex undergoes

photoreduction Light-dependent reactions is jargon for certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and ...

. In this process, the complex absorbs a

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

of light and subsequently decomposes to form and . The iron centre is reduced (gains an electron) from the +3 to the +2

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

, while an oxalate ion is oxidised to

: : 2 []³⁻ + ''hν'' → 2 []²⁻ + 2 + This reaction provides an efficient chemical method for Photometry (optics), photometry and

, the measurement of light and higher-energy electromagnetic radiation. Potassium ferrioxalate is over 1000 times more sensitive than

uranyl oxalate Uranyl oxalate (UO2C2O4) is a pale yellow powdered uranyl compound. It is often encountered in industrial nuclear processes at both the front and back-end of the nuclear fuel cycle. Due to its hygroscopicity, uranyl oxalate rarely exists in the deh ...

, the compound previously used for these purposes. While the complex itself is insensitive to

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

s, the

salt can be used to measure them. A lithium-6 nucleus can absorb a neutron and emit

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...

and a

triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus' ...

with high energies, which presumably decompose the nearby ferrioxalate.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.

References

{{reflist Iron complexes Anions Oxalato complexes Light-sensitive chemicals