The maximum hardness and minimum polarizability principles in accordance with the Bent rule

Energies, hardness values and polarizabilities for different isomers of SF₄, SF₄O molecules and a family of PCl_xF_5?x, (x=1,2,3,4) compounds are calculated at the density functional level of theory (B3LYP) using different basis sets (6-31G**, 6-311G**, 6-31++G and 6-311++G). For all molecules and in all cases the Bent rule is confirmed with the stability orders, which is obtained from the calculated energies. It is also found that for each molecule the isomer in which the more electronegative atom occupies the axial position has maximum hardness; and therefore according to the Maximum Hardness Principle (MHP), it must be the most stable isomer. This result is consistent with the Bent rule, but for some cases in which there are more than two isomers, the obtained trends for the stability are not in accordance with this rule. On the other hand, since the calculation of dipole polarizability is very sensitive to the quality of the selected basis set, only for those calculations in which diffuse basis sets are used the obtained trends of stability from the Minimum Polarizability Principle (MPP) are the same as those which are predicted from the Bent rule. It seems that when there are more than two isomers for a molecule both the MHP and MPP can only predict the most stable isomer and cannot predict the correct trend for the stability of the isomers.