The two alternative chemical bonding models for PCMs have been recently reviewed and it has been concluded that both metavalent and ERMB models describe the same bonding scenario. Both models agree that some electrons are localized, i.e. shared between two atoms, and some are delocalized. The two models also agree that the unconventional bonding in PCMs is characterized by sharing less than two electrons per atom pair. In this sense, supporters of the hypervalent ERMB model assume that the ERMB has only two bonding electrons shared between three centers, as was suggested for the Rundle-Pimentel 3c-4e model . Consequently, Jones has proposed that, although terms like resonant bonding and hypervalent bonding are well extended in scientific literature and the term metavalent bonding has become popular in the last five years, the unconventional bonding in PCMs should be called electronrich multicenter bonding since the existence of multicenter bonds was already postulated in the early days of quantum mechanics .