Although the concept of resonance and resonant bonding has been widely used and discussed to explain the unconventional bonding in PCMs, as recently reviewed , two alternative chemical bonding models have rivaled for explaining their exceptional properties in the last decade. On the one hand, several groups, including Kolobov and coworkers , Dronkowski and coworkers , and Lee and Elliot have considered that PCMs feature electronrich multicenter bonds (ERMBs). The ERMB is considered to be the generalization and extension of the three-center-four-electron (3c-4e) bond, proposed by Rundle and Pimentel for molecules, such as I3 ꟷ and XeF2 , and that is also related to the concepts of donor-acceptor bonds and hypervalent bonds . In particular, Dronkowski and coworkers demonstrated the electron delocalization and multicenter character of bonding in PCMs by calculations of the projected force constants along several atoms and of the crystal-orbital bond index (COBI), together with its integrated values for two-center and three-center bonds, ICOBI(2c) and ICOBI(3c), respectively . The multicenter character of bonding in PCMs has also been recently justified on the basis of a three-center "covalent-like" interaction .