Ab initio and DFT calculations indicate that the charge-localized forms (C-2v symmetry) of croconate (C5O52-) and rhodizonate (C6O62-) dianions cannot exist as a single entity. Instead, this form of the dianions can exist in the presence of counter cation(s) or in hydrogen-bonded networks. For the charge-delocalized (D-5h symmetry) form of C5O52-, it is found that the HF/3-21G(d) and CASSCF(n,m)/3-21G(d) levels of theory yield C-C and C-O bond lengths that are within the experimental range. However, for the D-6h tautomer of C6O62-, all the theoretical methods that have been tried led to optimized C-C bond lengths that are slightly (about 0.02-0.06 Angstrom) too long. (C) 2004 Elsevier B.V. All rights reserved.