The purpose of this review is to give an overview of three important N-bidentate ligands: 1,10-phenanthroline -phen-, 2,2′-bipyridine -bpy-, and ethylenediamine -en-. We have not attempted to be comprehensive because of the huge amount of activity being done in coordination chemistry using these ligands. Instead we present a full structural and geometrical study by using the Cambridge Structural Database -CSD- combined with theoretical calculations that allow us to parameterize their coordinating properties and ability to coordinate to transition and non-transition metals. More importantly, we illustrate that upon coordination and formation of the five-membered chelate ring, these ligands are able to adapt themselves to the requirements of the different metals by changing the MN distances and NMN angles. Therefore, a redefinition of the preferences of these ligands to metals with large ionic radii is needed. Finally, we will present some facts about the participation of these ligands in inorganic–organic hybrids -IOHs- based on Keggin polyoxometalates -POMs-.

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