Metal complexes of pyridoxal-based semicarbazone, thiosemicarbazone, and isothiosemicarbazone ligands (2005–2025): A comparative review of structure and biological activity

Pyridoxal-based semicarbazone, thiosemicarbazone, and isothiosemicarbazone ligands represent an important class of Schiff bases with significant potential in coordination chemistry and biomedical applications. Their ability to coordinate metal ions through multiple donor atoms, combined with tautomeric flexibility and variable protonation states, enables the formation of structurally diverse complexes. This review provides a comparative overview of metal complexes derived from these ligands reported in the period 2005–2025, with emphasis on the relationships among ligand structure, coordination behavior, and physicochemical and biological properties. The reported systems include a broad range of metal ions, predominantly first-row transition metals, as well as heavier transition metals and selected main-group and lanthanide ions, allowing the influence of metal-ion identity, oxidation state, coordination preferences, and redox properties to be comparatively assessed. The analysis highlights how variations in donor atoms, protonation state, and the presence of co-ligands influence geometry, stability, and intermolecular interactions. In addition, the biological activities of these systems, including anticancer, antimicrobial, and antioxidant effects, are discussed in relation to their interactions with biomolecular targets, such as DNA and serum proteins. This review identifies key trends in structure–property relationships and outlines perspectives for the rational design of new metal-based systems with improved selectivity and functional performance.