The equivalence of MMBs and EDMBs is further supported by the mechanism of formation of the MMBs and EDMBs in contrast to that of ERMBs. The ERMB formation in I3 ꟷ can be considered as the result of the approach of an I ꟷ ion to an I2 molecule (Fig. ) . As they approach, a trans influence the first unit into the second one occurred. According to the donor-acceptor charge-transfer model, a charge transfer from the donor unit (I ꟷ ) to the acceptor unit (I2) occurs as they approach each other; in other words, the antibonding orbital of the covalent bond of the I2 molecule is populated due to the LEP of the I ꟷ ion. This trans influence occurs until both the I-I distance of the I2 molecule and the intermolecular I-I distance between the I ꟷ ion and one of the I atoms in the I2 molecule become almost equal . The mechanism of ERMB formation can be also understood in the light of the -hole bond model . In this model of secondary donor-acceptor bonds, the I ꟷ ion with electronegative charge (nucleophilic) is attracted by the electropositive (electrophilic) region of the -hole (region of low electronic density) formed at the end of the I-I covalent bond of the I2 molecule. When both electrophilic and nucleophilic regions approach, there is no charge transfer from the donor to the acceptor unit, as claimed in the charge-transfer model, but simply a polarization of the charges in the I2 molecule, which are induced by the approach of the I ꟷ ion. This means that as the I ꟷ ion approaches the closer I atom of the I2 molecule, the ion shares part of its charge with the atom of the molecule and this sharing induces a shift of part of the charge of the closer I atom of the I2 molecule towards the other I atom of the I2 molecule, i.e. the I atom that is opposite to the I ꟷ ion. This way the central I atom avoids severely violating the octet rule and a symmetric (─ + ─) charge configuration is observed in the 3c-4e bond that explains the more electronegative external parts of this multicenter bond and also the large ET values of molecules with ERMBs. A similar procedure allows us to explain the linear ERMB formation and charge distribution in other linear molecules, such as XeF2 and FHF ꟷ . This bonding model is schematized in Fig. . Therefore, it can be concluded that the ERMB formation does not involve a charge transfer between the primary and secondary bonds, as in MMBs, i.e. the primary bond in the ERMB does not lose charge, thus resulting in a rather large ES value compared to MMBs .