The croconate anion also forms compounds with trivalent cations such as aluminium (yellow), chromium (brown), and iron (purple). These compounds also include hydroxyl groups as well as hydration water and have a more complicated crystal structure. No indication was found of sandwich-type bonds between the delocalized electrons and the metal (as are seen in ferrocene, for example), but the anion can form metal complexes with a large variety of bonding patterns, involving from only one to all five of its oxygen atoms.
