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 not) le gas (Ng)/methyl cation complexes [Ng-CH3-Ng](+) (Ng = He and Ne), which feature a delocalized D-3-symmetric Structure with two equivalent C-Ng bonds? From Ng=Ar onwards, the [Ng-CH3-Ng](+) complex again acquired a propensity to localize one of its axial C-Ng bonds and to largely break the other one, and this propensity increases ill the order Ng = Ar < Kr < Xe < Rn. The behavior of the helium and neon complexes violates the ball-in-a-box principle. Why does this happen? The purpose of this stuck, is to answer these questions and to understand why carbon can become truly hypervalent under certain conditions. To this end, we have Carefully analyzed the structure and bonding in NgCH(3)Ng(+) and, for comparison. CH(3)Ng(+), NgHNg(+), and NgH(+). It appears that, at variance with [Cl-CH3-Cl](-), the carbon atom in [Ng-CH3-Ng](+) call no longer be considered as a ball in a box of the five substituents.