**4. Nitrogen, Oxygen, and Sulfur Binding Sites**

In several tridentate structures, an oxygen atom was replaced by a, *S* donor binding site. Sulfur–nitrogen chelating agents are employed for their marked biological activities both as ligand and in their transition metal complexes. In many cases, the versatile C=N bond and aromatic heterocycle rings were employed in the ligand construction. Many *N*,*N*,*S* tridentate zinc complexes were explored by paying attention to both structural and spectroscopic behavior. More rarely, *S*,*N*,*S* tridentate pincer ligands with zinc salts [124] were explored, mainly screened for their reactivity and/or catalytic activity rather than for the PL properties. On the other hand, a few significative examples of mixed *N*,*S*,*O* binding sites were recently proposed. In most cases, interest was focused on the X-ray structural exploration of the coordination core and in their basic chemo-physical properties. In some cases, the observation of specific spectroscopic properties promoted the investigation of the emission properties and even moved an applicative interest.
