A Unified Theory of Electron-Rich and Electron-Deficient Multicenter Bonds in Molecules and Solids: A Change of Paradigms

Here, we propose a multicenter bond theory that addresses the origin and mechanisms behind the formation of electron-rich multicenter bonds (ERMBs) and electron-deficient multicenter bonds (EDMBs), with special emphasis on molecules and solids primarily composed of electron-rich elements. We show that both types of multicenter bonds have the same origin, but a different mechanism of formation upon increasing electronic density; e.g., upon reduction, increase of pressure, or chemical substitution of elements by their heavier analogs. In addition to our recent demonstration of the formation of EDMBs in electron-rich systems, such as pnictogens, chalcogens, and chalcogenides related to phase change materials (see H.H. Osman et al., J. Mater. Chem. C 2024, 12, 10447), here we present other examples of solids with electron-rich elements forming EDMBs and ERMBs. We conclude that EDMBs can occur not only as three-center-two-electron (3c-2e) bonds in molecules but also as linear or quasi-linear two-center-one-electron (2c-1e) bonds in extended solids. In addition, we propose that pure ERMBs can only occur as linear or quasi-linear three-center-four-electron (3c-4e) bonds. All these claims suppose a change of paradigms regarding the current understanding of ERMBs and EDMBs. To understand the formation of ERMBs and EDMBs in electron-rich elements, we show some of the simplest geometries of linear EDMBs and ERMBs along one (1D), two (2D), and three (3D) dimensions that can be found in the hypercoordinated multicenter units of molecules and solids with electron-rich elements and propose a new way of notation of these hypercoordinated units. Finally, we show that both types of multicenter bonds, in general, do not violate the doublet/octet rule.