Transition metal oxalate complexes are
coordination complex
A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...
es with
oxalate
Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as ...
(C
2O
42−)
ligand
In coordination chemistry, a ligand is an ion or molecule with a 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 el ...
s. Some are useful commercially, but the topic has attracted regular scholarly scrutiny. Oxalate (C
2O
42-) is a kind of dicarboxylate ligand. As a small, symmetrical dinegative ion, oxalate commonly forms five-membered MO
2C
2 chelate rings. Mixed ligand complexes are known, e.g.,
o(C2O4)(NH3)4sup>κ+.
Examples
Homoleptic complexes
Homoleptic oxalato complexes are common, e.g., those with the formula
(κ2-C2O4)3sup>n-: M = V(III), Mn(III), Cr(III), Tc(IV), Fe(III), Ru(III), Co(III), Rh(III), Ir(III). These anions are chiral (D
3 symmetry), and some have been resolved into their component enantiomers. Some early metals form tetrakis complexes of the type
(κ2-C2O4)4sup>n- M = Nb(V), Zr(IV), Hf(IV), Ta(V),
The Δ and Λ enantiomorphs of have been separated.
Oxalate is often a bridging ligand forming bi- and polynuclear complexes with (κ
2,κ'
2-C
2O
4)M
2 cores. Illustrative binuclear complexes are
2(C2O4)5sup>2- M = Fe(II) and Cr(III)
Mixed ligand complexes
Whereas homoleptic complexes are easier to describe, far more abundant are complexes with oxalate and other ligands. Many metals form polynuclear complexes with oxalate and water. In ferric oxalate, , one oxalate is bonded through all four oxygen atoms and another oxalate binds through only two oxygen atoms, in both cases
bridging. "Durrant's salt" contains the anionic complex .
Reactions and applications
Metal oxalate complexes are photoactive, degrading with loss of
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
. This reaction is the basis of the technique called
actinometry
An actinometer is an instrument that can measure the heating power of radiation. Actinometers are used in meteorology to measure solar radiation as pyranometers, pyrheliometers and net radiometers.
An actinometer is a chemical system or physic ...
.
Ferrioxalate
Ferrioxalate or trisoxalatoferrate(III) is a trivalent anion with formula . It is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions anions acting as ligands.
The ferrioxalate anio ...
undergoes
photoreduction
Light-dependent reactions are certain photochemical reactions 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 the second occurs ...
. 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 Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
, while an oxalate ion is oxidised to
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
:
: 2 []
3− + ''hν'' → 2 []
2− + 2 +
The redox reaction has been used to access unusual complexes. UV-irradiation of Pt(C
2O
4)(PPh
3)
2 gives derivatives of Pt
0(PPh
3)
2.
Metal oxalates with the stoichiometry 1:1 are often insoluble. This fact provides a way to separate metal ions from solutions, including extract of ores. Combustion of metal oxalates gives metal oxides.
Natural occurrence
The minerals
moolooite
Moolooite is a rare blue-green mineral with the formula Cu(C2O4)·n(H2O) (n<1) ( copper(II) oxalate hydrate). It was discovered in Bunbury Well, ...
and
antipinite
Antipinite ( IMA symbol: Atp) is a rare alkali Copper oxalate mineral with the chemical formula . Its type locality is the Tarapacá Region in Chile.Transition metal carboxylate complex
Transition metal carboxylate complexes are coordination complexes with carboxylate (RCO2−) ligands. Reflecting the diversity of carboxylic acids, the inventory of metal carboxylates is large. Many are useful commercially, and many have attracted ...
*
Oxalatonickelate The oxalatonickelates are a class of compounds that contain nickel complexed by oxalate groups. They form a series of double salts, and include clusters with multiple nickel atoms. Since oxalate functions as a bidentate ligand it can satisfy tw ...
References
{{oxalates
Ligands
Oxalato complexes