Before analyzing the mechanism of MMB formation in group-15 and-16 elements, it must be noted that the scheme of Fig. also conforms to the progressive formation of hypervalent ERMBs in molecules already commented on by Hoffmann and coworkers and recently reviewed . It has been suggested that the formation of ERMBs occurs due to the existence of a mixture of primary bonds and secondary donor-acceptor bonds. These secondary bonds, apart from van der Waals interactions between LEPs of different atoms and other possible minor interactions, are considered to be caused by the interaction of the LEP (donor) of atom C in Fig. and the antibonding orbital (σ * , acceptor) associated to the primary covalent ppσ-bond between A and B atoms in Fig. . This means that secondary donoracceptor bonding is due to the interaction of two antibonding orbitals since the LEP, with assumed nonbonding character, has in fact a certain antibonding character due to s-p mixing , as confirmed by pCOHP calculations later discussed concerning Fig. . Many works have reported that the strength of the secondary bond is determined by the proper alignment of the directions of the LEP and σ*, which in turn depends on the d2/d1 ratio. In this way, the closer the A-B-C angle to 180 and the smaller the d2/d1 ratio the stronger the LEP-σ* interaction. This consideration suggests that, in general, the strength of the secondary bond is higher in stage 2 (Fig. ) than in stage 1 (Fig. ) due to the trans influence between primary and secondary bonds. In the following, we will show that the pressure-induced transformation from the pre-MMB scenario to the MMB scenario proceeds via three stages (Fig. ) in group-15 elements and via two stages (Fig. ) in group-16 elements. The only exception is Po, which is already in stage 3 (Fig. ), as already mentioned. It must be stressed that the above-described model of secondary LEP-σ* donor-acceptor interaction, which leads to the formation of hypervalent molecules, is a key concept of supramolecular chemistry that has been extensively revisited in the last two decades.