A bottom-up perspective on multicenter bonding in discrete molecular systems

Abstract

The understanding of the properties of advanced materials requires a deep knowledge of chemical bonding beyond the classical chemical bonding picture of covalent, ionic, and metallic bonds. For this purpose, we need to include multicenter bonds into the equation. Here, we approach multicenter bonds in discrete molecular systems by resorting to experimental structural data of finite non-branched chain-like molecules and ions formed by main-group non-metallic elements (X) with s- and p-type valence electrons, as obtained from the Cambridge Structural Database. Using very basic concepts and methods, we demonstrate that multicenter bonds in Xn molecules with n ≥ 3 can be understood with the help of current knowledge of two-center bonds. We evidence that some of the studied chains exhibit covalent bonds, while others exhibit two types of multicenter bonds: electron-rich multicenter bonds (ERMBs), best known as three-center four-electron (3c–4e) bonds or hyperbonds, and electron-deficient multicenter bonds (EDMBs), whose better known example is the three-center-two-electron (3c–2e) bond. Interestingly, linear Xn molecules for n > 3 do not exhibit ERMBs, but “hybrid” multicenter bonds, which consist of a concatenation of ERMBs and EDMBs, where the ratio of EDMBs to ERMBs increases with n; i.e., the electron deficiency in the chain increases with n. Consequently, infinite linear atomic chains feature exclusively EDMBs (even for electron-rich elements). The above observations suggest that there is an inherent limitation to the formation of linear atomic molecules for n > 3 with only ERMBs. All these observations fully agree with the recently proposed unified theory of multicenter bonding, which suggests that multicenter bonds are the missing link between weak secondary bonds and strong primary bonds and promotes a unitary vision of chemical bonds (both in molecules and solids).