Oxalate - Structure

Structure

X-ray crystallography of simple oxalate salts show that the oxalate anion may adopt either a planar conformation with D_2h molecular symmetry, or a conformation where the O-C-C-O dihedrals approach 90° with approximate D_2d symmetry. Specifically, the oxalate moiety adopts the planar, D_2h conformation in the solid-state structures of M₂C₂O₄ (M = Li, Na, K). However, in structure of Cs₂C₂O₄ the O-C-C-O dihedral angle is 81(1)°. Therefore, Cs₂C₂O₄ is more closely approximated by a D_2d 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−. 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. 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.