Croconic acid metal complexes

Of the known cyclic oxocarbon acids, the systems based on squaric (68) and croconic (69) acids have been most widely studied. The loss of two protons from these acids gives rise to aromatic dianions as shown in equations (18) and (19), and these can coordinate to metal anions in a variety of ways. Unidentate coordination (70,77) is known for both systems but is not common. Simple bidentate chelate coordination (78) is also relatively uncommon but is observed in a number of croconate complexes. The squarate anion adopts this mode only with larger cations, such as the group 2 and lanthanide metals, and then only in association with additional bridging interactions. Bridging coordination modes dominate the chemistry of these anions, some of which are shown here (71-76), (79-81). The various modes of coordination can usually be distinguished by IR spectroscopy, and the use of NMR spectroscopy has also been investigated. 

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