The Oxalate Dianion, C2O42-: Planar or Nonplanar?

Theoretical studies have shown that for the isolated oxalate dianion, C2O42-, which contains a long C-C single bond, the conformer with a 90 degrees twist about the C-C bond (D-2d symmetry) is more stable than the planar (D-2h) conformer that is almost invariably drawn for the dianion. The D-2d 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 degrees of the D-2d conformer. This indicates that bonding or "crystal packing" forces usually outweigh the 2-6 kcal/mol barrier calculated for rotation between the two conformers.