Traditionally, the LEP-σ * interaction has been explained as a charge-transfer model in which part of the charge is transferred from the LEP donor to the σ * acceptor. This trans influence is accompanied by an increase in the bond distance of the covalent bond. This interaction has been reinterpreted in the last decades in light of the σ-hole concept for secondary bonding . According to the -hole bond model, the secondary donor-acceptor bond consists of an electrostatic interaction between an electrophilic (acceptor-like) moiety and a nucleophilic (donor-like) moiety. This Coulombic interaction is related to the more electropositive potential of the acceptor moiety (-hole) and the more electronegative potential of the donor moiety that are in turn related to their different electronic densities. In our work, the positive electrostatic potential (-hole) would be located at both ends of the primary covalent pp-bond (e.g. As-As) and would correspond to the antibonding orbital, while the negative electrostatic potential is located at the LEP. A key difference between the two models is that in the -hole bond model there is no net charge transfer from the donor to the acceptor, but a polarization or charge shift in the acceptor moiety, which is induced by the proximity of the donor moiety. The two models of charge-transfer and -hole for secondary bonding in group-15 and -16 elements will be further discussed in the next section once the mechanism of MMB formation in these families is clarified. Now let us see the mechanism of formation of MMBs in these two elemental families.