Hypervalent versus Nonhypervalent Carbon in Noble-Gas Complexes

Silicon in [ClSiH3Cl]− is hypervalent, whereas carbon in [ClCH3Cl]− is not. We have recently shown how this can be understood in terms of the ball‐in‐a‐box model, according to which silicon fits perfectly into the box that is constituted by the five substituents, whereas carbon is too small and, in a sense, “drops to the bottom” of the box. But how does carbon acquire hypervalency in the isostructural and isoelectronic noble gas (Ng)/methyl cation complexes [NgCH3Ng]+ (Ng=He and Ne), which feature a delocalized D3h‐symmetric structure with two equivalent CNg bonds? From Ng=Ar onwards, the [NgCH3Ng]+ complex again acquires a propensity to localize one of its axial CNg bonds and to largely break the other one, and this propensity increases in the order Ng=Ar