Potassium croconate

Another example is afforded by potassium croconate K2C5O5, which was isolated and characterized as K2C5O5. 2H2O by Dunitz and subsequently by us. The coordination around the potassium cation is depicted in Fig. 9a. The croconate dianions form columns, extending parallel to the c-axis. completely surrounded by cations and water molecules, with these latter acting as "pinchers" on the dianions (see Fig. 9b.c). The short interplanar separation (3.30 A) is thus a result of the external interactions that "compress" the anions together. 

The deconstruction of the potassium croconate crystal allows us to recognize the analogies with another member of the family of oxocarbon anions, rubidium rodizhonate Rb2C606. The crystals possess some remarkable... 

Dunitz, J.D. Seiler. P. Czechtizky. W. Crystal structure of potassium croconate dihydrate, after 175 years. Angew. Chem.. Int. Ed. 2001. 40. 1779-1780. 

Acetylene was obtained by Edmund Davy by the action of water on the residue from the preparation of potassium by heating potassium carbonate and carbon (see p. 95), and Gmelin from a product of the same process obtained croconic acid, later recognised as a derivative of benzene (see p. 794). These were also synthetic products. 

The cochineal-coloured solid was examined by J. F. Heller, who showed that it is the potassium salt of what he called rhodizonic acid (poSi ojy I colour rose-red), and he also examined croconic acid. His results were mostly confirmed by A. Werner. Liebig found that rhodizonate of potash dissolved in water forms the croconate and oxalate, and hence he suggested the formula 504 + 0203 = 0707 for rhodizonic anhydride. For rhodizonate of potash he... 

Nietzki and Benckiser confirmed the formula C5O5H2 for croconic acid and were able to examine a specimen of the potassium salt which had been prepared by Gmelin and was still in Heidelberg. Since it forms an acid salt,... 

The electronic spectra of the potassium salts of squarate 2, croconate 3, and rhodizonate (4) ions in aqueous solution exhibit bands with high molar absorptivity, in the order of 10 1 mol cm , and a similar profile for all the spectra, with unfolded bands (Figure 5.2). 

The single-crystal X-ray analysis of potassium squarate monohydrate, determined in 1964, confirmed its symmetry [15cj. The D51, symmetry of the croconate ion 3 was confirmed in the same way at about the same time. 

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