The Oxalate Dianion, C2O42–: Planar or Nonplanar?

Abstract

Theoretical studies have shown that for the isolated oxalate dianion, C2O42–, which contains a long C–C single bond, the conformer with a 90° twist about the C–C bond (D2d symmetry) is more stable than the planar (D2h) conformer that is almost invariably drawn for the dianion. The D2d conformer has been shown to persist in aqueous solutions of simple oxalate salts. However, in oxalato complexes or in solid oxalates, the O–C–C–O dihedral angle for C2O42– is commonly observed to be quite different from the 90° of the D2d conformer. This indicates that bonding or “crystal packing” forces usually outweigh the 2–6 kcal/mol barrier calculated for rotation between the two conformers.