Metal ions are of great significance to the environment and biological system. Particularly, trivalent - iron, aluminium and chromium - and divalent - copper, mercury, cadmium, and zinc - cations are some of the more important. Among the different chemosensors, those based on ion-induced changes in fluorescence are especially suitable as they are easy to use and usually give an instantaneous response with high sensitivity.

Recently we have designed and synthesized two new ligands derived from bis-vanillin. Both ligands were studied as fluorimetric sensors of trivalent cations. It was concluded that a combination of them can be used to discriminate between Fe³⁺, Al³⁺ and Cr³⁺.

Based in these results, we present a theoretical study based on the Functional Density Theory related to the complexation types of both ligands with M³⁺ cations, which structures contained a hydrazoline or isoniazide moiety. Moreover, we realize a predictive theoretical study to explore the ligands' complexation with M²⁺ cations.  The optimized structures of the ligands and their cation complexes are presented and discussed. In addition, for the M³⁺ complexes, we analyze the changes in the dihedral angle of the complexes structures and their connection with the experimental observations. The global energies of the optimized structures show that the complexes possess less energy than the isolated ligands. On the other hand, in all the cases the values ​​of the difference between HOMO and LUMO energies are determined to compare the energy gap and relate them to the effects observed in the UV bands.