Two concomitant polymorphs and two isomorphous forms with different chemical compositions, which transform into the same substance upon thermal treatment

The organometallic cationic acid [Co-III(eta(5)-C5H4COOH)(2)](+) (AC) is crystallised from hydrochloric acid solutions in tow polymorphic modifications of the salt [Co-III(eta(5)- C5H4COOH)(2)]Cl.H2O, ACl.H2Ocube and ACCl.H2Osword. On the other hand, ACCl.H2Osword is isomorphous wit the triply-hydrated zwitterion [Co-III(eta(5)- C5H4COOH)eta(5)-C5H4COO)], ZW.3H(2)O. ACCl.H2Oswords ACCl.H2Ocube and ZW.3H(2)O are all converted into the anhydrous form [Co-III(eta(5)-C5H4COOH)(eta(5)-C5H4COO)], ZW, upon thermal treatment. The sructura relationship between the ionic and neutral systems and their interconversio have been investigated with a combination of solid-state techniques (single crystal and powder X-ray diffraction, thermogravimetric analysis, mass spectrometry and solid state C-13 CP/MAS NMR).