IntroductionGmelin reported in 1825 the preparation of the ion C5O52−and of the corresponding acid H2C5O5. This acid he called croconic acid because of its yellow colour.1The dianion C5O52−belongs to the family of oxocarbon anions that include the deltate, squarate, and rhodizonate anions.2The interest in these anions was originated by the view that cyclic oxocarbon anions of the general formula CnOn2−might show aromaticity stabilised by electron delocalisation of the π electrons around the ring.3It is worth recalling that the isolation by Gmelin of croconic acid and of the croconate dianion is contemporary to the isolation by Faraday of benzene, the prototype of aromatic molecules. While the solid state structures of deltic4and squaric acid5were established a long time ago, those of the heavier members remain unknown. What could be the reason for this ‘absence’ from the vast repertoire of organic molecules characterised by diffraction methods6is hard to say. The crystallisation of croconic acid by us is the result of our systematic quest for new building blocks to be utilised in the crystal engineering7of hybrid organic–organometallic materials.8