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The theoretical study of α-, β-, γ-, δ-, ε- MCl2(Azpy)2 isomers with (M = Fe, Os and Ru) complexes is carried out using Density Functional Theory (DFT) at the B3LYP / LANL2DZ level. This study is focused not only on the effect of metals over geometric, electronic and reactivity parameters, but also on their anti-cancer effect. Its results that the geometric parameters undergo small modifications. These modifications evolve from iron to osmium through ruthenium complexes. Thus, the lengths of the bonds M-X (with X = Cl, N2, Npy) follow the following order Fe-X <Ru-X <Os-X. However, regarding their angular variation that undergoes deformation through the octahedron shape, it could be related to Jahn Teller effect. Also, the substitution of Ru by Os would increase the reactivity of these complexes. Among the isomers studied, the ε-Fe, δ-Ru and δ-Os complexes are likely to bind easily to the DNA. The values of the dipole moments are arranged in the following order: μ (ε-M)> μ (β-M)> μ (α-M)> μ (γ-M)> μ (δ-M) within these azopyridine complexes. Finally, we notice that the substitution of Ru by Os improves the cytotoxicity and the fluorescence of these complexes. The δ-Os isomer has the best cytotoxic and photosensitive characteristics of these azopyridine complexes and would be the ideal isomer for the diagnosis and treatment of cancers.