Metal complexes incorporating tridentate ONO pyridyl hydrazone Schiff base ligands: Crystal structure, characterization and applications

Schiff bases are an extensive group of materials which have a carbon–nitrogen double bond (–C = N–). The present review gives a perspective on this class of compounds by highlighting the significance of various types of tridentate ONO pyridyl Schiff base ligands and their corresponding metal complexes. The review focuses on the reactions of salicylaldehyde and its derivatives with different acylhydrazides, such as picolinohydrazide, nicotinohydrazide and isonicotinohydrazide, in which the single crystal structures of the resulting products have been reported in the last two decades (2002–2022). This class of Schiff bases show amide-iminol tautomerism, in which the carbon–nitrogen double bond is a component of the iminol tautomer. The molecular structure, spectral analysis and applications of ONO pyridyl Schiff base complexes are covered in this article.