Coordination modes of 5-pyrazolones: A solid-state overview

The coordination modes of 5-pyrazolones are reviewed in light of the available X-ray diffraction studies of their complexes. This structural analysis firstly concerns the coordination behaviour of the molecules without any donor atoms other than those associated with the pyrazolone ring, and then deals with substituted pyrazolones in which the substituent provides additional donor atoms. Although the C4-substituted molecules are the most widely explored because they are very versatile ligands, interest in the N(1)-substituted derivatives and in the bis- and poly-5-pyrazolones has been growing steadily in recent years.