Metavalent multicenter bonding in pnictogens and chalcogens: nature and mechanism of formation

Abstract

Due to the exceptional property portfolio and technological applications of phase change materials, mostly chalcogens related to IV-VI and V2-VI3 families, which are in turn related to pnictogens (group V or 15) and chalcogens (group VI or 16), the nature of the unconventional chemical bonding in these materials has been debated for almost 70 years. This unconventional bond, which has been quoted in the literature as resonant, hypervalent, electron-rich multicenter, three-center-four-electron (3c-4e), and metavalent, is believed to be responsible for the exceptional properties of phase change materials. In the last decade, two bonding models, the metavalent and the electron-rich multicenter models, have competed to explain the nature of this unconventional bond, which we have here renamed as metavalent multicenter bond (MMB) for the sake of clarity. In this comprehensive work, we address the nature of MMB and propose that MMB is an electron-deficient multicenter bond (EDMB), related to the threecenter-two-electron (3c-2e) bond. For that purpose, we explore the pressure-induced mechanism of MMB formation in the some of the simplest possible systems, pnictogens (As, Sb, Bi) and chalcogens (Se, Te, Po), with density-functional theory calculations. In the way, we find that polonium is the only element among chalcogens and pnictogens with crystalline α and β structures already exhibiting MMBs at RP. We find that the mechanism of MMB formation in pnictogens (chalcogens) is comprised of three (two) stages, is similar to that of the EDMB formation in B2H6, in some Zintl phases, intermetallics, and cluster compounds, and in atomic/polymeric nitrogen and hydrogen at high pressures. On the other hand, the mechanism of EDMB formation is completely different from that of the 3c-4e bond formation in molecules. Finally, we propose the simplest geometries of EDMBs that can be found in solids along one, two, and three dimensions and comment on the validity of the doublet/octet rules in the hypercoordinated multicenter units with EDMBs.

Electron-deficient multicenter bonding in phase change materials: A chance for reconciliation

“…As said above, this position is at odds with the ES values recently calculated for both molecules and solids with ERMBs and EDMBs and plotted in Fig.2 [4,12].In summary, we consider that the reconciliation of both metavalent and hypervalent bonding models can be reached if: i) the supporters of the metavalent bonding model take into account the multicenter character of bonding in PCMs, and ii) the supporters of the hypervalent bonding model take into account the electron-deficient character of bonding in PCMs. Taking into account i) and ii) the metavalent and hypervalent bonding models lead naturally to the electron-deficient multicenter bonding model proposed by Manjón and coworkers[12].In relation to the above comments, we consider that the reconciliation of both metavalent and hypervalent bonding models is easier for the metavalent than for the hypervalent bonding models. The reason is that the multicenter character of the bonds in PCMs and also in related materials has been enough justified in previous works[12,22,24,45]; however, the formation of electron-deficient bonds in molecules and solids composed of electron-rich elements seems not to be so justified.…”

“…Manjón and coworkers have recently made public a work in which they have evaluated both the perspectives of the hypervalent and metavalent bonding models in PCMs and have considered the pros and cons of both bonding models [12]. As a result, Manjón and coworkers have come to the conclusion that PCMs feature electron-deficient multicenter bonds; i.e.…”

“…On the other hand, the ES value of EDMBs is smaller than that of single covalent bonds of comparable polarity. Unlike the supporters of the metavalent bonding model, Manjón and coworkers have shown that the term metavalent is unnecessary to describe bonds in PCMs since they are electron-deficient, such as the bridged B-H-B bonds in B2H6 and the bonds in sc-As (or sc-Sb) [12]. Manjón and coworkers consider that metallic bonding is indeed characterized by delocalized electrons and can be qualified as an electron-deficient bond since it is the last step in the process of electron delocalization.…”

“…On the other hand, the success of the hypervalent (electron-rich multicenter) bonding model resides in the discovery of the multicenter character of the bonds in PCMs that leads to hypercoordinated linear bonding motifs. However, Manjón and coworkers have shown that these linear motifs can be found in both electron-rich and electrondeficient molecules and solids [12]. Therefore, the term "hypervalent" is no longer valid and should be replaced by "hypercoordinated".…”

Electron-Deficient Multicenter Bonding in Phase Change Materials: A Chance for Reconciliation

“…For many years, the chemical bonding in PCMs and related materials, such as pnictogens and chalcogens (note that pnictogens are isoelectronic to A IV B VI compounds, some of them being PCMs [ 5 , 6 , 7 , 8 ]), was considered to be a rare case of covalent bonding in which a resonant bonding of p-type orbitals occurred [ 5 , 6 , 7 , 8 ]. In the last decade, the consensus on the chemical bonding in PCMs has been broken, as shown by the recently published papers [ 4 , 9 , 10 , 11 , 12 , 13 , 14 ].…”