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The structure of the oxalate anion

A ball-and stick model of oxalate
Oxalate (IUPAC: ethanedioate) is the dianion with the formula C2O42−, also written (COO)22−. Either name is often used for derivatives, such as salts of oxalic acid (for example disodium oxalate, (Na+)2C2O42−) or esters thereof (for example dimethyl oxalate, (CH3)2C2O4). Oxalate also forms coordination compounds where it is sometimes abbreviated as ox. Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, the primary constituent of the most common kind of kidney stones. Contents * 1 Relationship to oxalic acid * 2 Structure * 3 Occurrence in nature * 3.1 Physiological effects * 4 As a ligand * 5 Safety * 6 See also * 6.1 Raphides * 6.2 Oxalate salts * 6.3 Oxalate complexes * 6.4 Oxalate esters * 7 References| Relationship to oxalic acid

The dissociation of protons from oxalic acid proceeds in a stepwise manner as for other polyprotic acids. Loss of a single proton results in the monovalent hydrogenoxalate anion HC2O4−. A salt with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant (Ka) for loss of the first proton is 5.37×10−2 (pKa = 1.27). The loss of the second proton, which yields the oxalate ion has an equilibrium constant of 5.25×10−5 (pKa = 4.28). These values imply that, in solutions with neutral pH, there is no oxalic acid, and only trace amounts of hydrogen oxalate.[1] The literature is often unclear on the distinction between H2C2O4, HC2O4-, and C2O42-, and the collection of species is referred to oxalic acid. Structure

X-ray crystallography of simple oxalate salts show that the oxalate anion may adopt either a planar conformation with D2h molecular symmetry, or a conformation where the O-C-C-O dihedrals approach 90° with approximate D2d symmetry.[2] Specifically, the oxalate moiety adopts the planar, D2h conformation in the solid-state structures of M2C2O4 (M = Li, Na, K).[3] However, in structure of Cs2C2O4 the O-C-C-O dihedral angle is 81(1)°.[4][5] Therefore, Cs2C2O4 is more closely approximated by a D2d symmetry structure because the two CO2 planes are staggered. Interestingly, two forms of Rb2C2O4 have been structurally characterized by single-crystal, X-ray diffraction: one contains a planar and the other a staggered oxalate. As the preceding examples indicate that the conformation adopted by the oxalate dianion is dependent upon the size of the alkali metal to which it is bound, some have explored the barrier to rotation about the central C−C bond. It was determined computationally that barrier to rotation about this bond is roughly 2–6 kcal/mole for the free dianion, C2O42−.[6] Such results are consistent with the interpretation that the central carbon-carbon bond is best regarded as a single bond with only minimal pi interactions between the two CO2 units.[2] This barrier to rotation about the C−C bond (which formally corresponds to the difference in energy between the planar and staggered forms) may be attributed to electrostatic interactions as unfavorable O−O repulsion is maximized in the planar form. It is important to note that oxalate is often encountered as a bidentate, chelating ligand, such as in Potassium ferrioxalate. When the oxalate chelates to a single metal center, it always adopts the planar conformation. Occurrence in nature

Oxalate occurs in many plants, where it is synthesized via the incomplete oxidation of carbohydrates. Oxalate-rich plants include fat hen ("lamb's quarters"), sorrel, and several Oxalis species. The root and/or leaves of rhubarb and buckwheat are high in oxalic acid.[7] Other edible plants that contain significant concentrations of oxalate include—in decreasing order—star fruit (carambola), black pepper, parsley, poppy seed, amaranth, spinach, chard, beets,...
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