Hypervalent silicon versus carbon: Ball-in-a-box model

Why is silicon hypervalent and carbon not? Or why is [Cl-CH ₃-Cl]^- labile with a tendency to localize one of its axial C-Cl bonds and to largely break the other one, while the isostructural and isoelectronic [Cl-SiH₃-Cl]^- forms a stable pentavalent species with a delocalized structure featuring two equivalent Si-Cl bonds? Various hypotheses have been developed over the years focusing on electronic and steric factors. Here, we present the so-called ball-in-a-box model, which tackles hypervalence from a new perspective. This model reveals the key role of steric factors and provides a simple way of understanding the above phenomena in terms of different atom sizes. Our bonding analyses are supported by computation experiments in which we probe, among other things, the shape of the S_N2 potential-energy surface of Cl^- attacking a carbon atom in the series of substrates CH₃Cl, •^.CH ₂Cl, ..CHCl, and ...CCl. Our findings for ClCH₃Cl ^- and ClSiH₃Cl^- are generalized to other Group 14 central atoms (Ge, Sn, and Pb) and axial substituents (F).