Crystallization from hydrochloric acid affords the solid-state structure of croconic acid (175 years after its discovery) and a novel hydrogen-bonded network

The structure of croconic acid is determined on single crystals grown from a solution of HCl 1 M; it is shown that O–H⋯O interactions organize the H2C5O5 molecules in an ‘accordion’ type supramolecular arrangement of hydrogen-bonded molecules; the stacking of [Cl]−·H2C5O5 units in the organometallic co-crystal [(η5-C5H5)2Co][Cl]·H2C5O5 is also analysed.