The structural properties and the first ionization potential (I1) of the clusters Mo6X14 (X = F, Cl, Br, I) at 24 metal electrons (ME) were studied by the density functional method at the theoretical level PBE/TZ2P. The molecular orbitals (MO) diagrams of molybdenum halide clusters what their structures have been optimized in the most stable Oh point group symmetry show that whatever halogen, a large energy gap separates the occupied levels from the vacant levels for an ME count equal to 24. This large energy gap increases from fluorine to chlorine then decreases from chlorine to iodine. The optimized structures show that the structural variations depend on the nature of the halogen. The evolution of the optimized geometries of the clusters is in agreement with the evolution of the experimental geometries, namely an expansion of the octahedral core and an increase in distances metal-ligand when the halogen is heavier. These changes are linked to the steric hindrance of halogens. This agreement is better when the scalar relativistic effects  and the spin-orbit coupling are taken into account. The calculations of ionization energy have shown that the lowest calculated I1 is that of the cluster [Mo6F14]2-. This certainly plays a role in the inability to isolate this cluster in fact. Other clusters have higher l1 in which I1 of the cluster [Mo6Cl14]2- being the greatest. This proves that it is more difficult to oxidize cluster [Mo6Cl14]2-. The evolution of first ionization potential calculated between the three halogenated species Cl, Br, and I vary depending on whether relativistic effects are taken into account. In agreement with the trend observed experimentally, it is essential to consider the scalar relativistic effects at least. The consideration the spin-orbit coupling exhibits a significant impact on the energy of the cluster  [Mo6I14]2-.