Crystal Structure of Potassium Croconate Dihydrate, after 175 Years

Stacks of croconate dianions separated by K+ ions and H2O molecules: the crystal structure of potassium croconate dihydrate, K2(C5O5)⋅2 H2O, isolated and described by Leopold Gmelin more than 170 years ago, has now been established by X-ray analysis (see picture). Since the anions have a formal charge of −2 and an interplanar separation of only 3.30 Å, the structure is not easily explained in terms of a simple ionic model.