Stable pentacoordinate carbocations: structure and bonding.

We report that only elements more electropositive than carbon (Group 13, 14, and Be) form stable symmetrical HnE--CH3--EHn + structures (E=Group 1, 2, 13, or 14 element) with a planar CH3 group symmetrically bonded to two EHn moieties, in analogy with prototypical SN2 transition structures. Analysis of the bonding situation of these pentacoordinate carbon molecules was studied by means of an energy decomposition analysis (EDA) of the interaction energy. This shows that HnE--CH3--EHn + molecules can be viewed as being composed of one CH3 group that is sigma-covalently bonded to two EHn groups forming a three-center, two-electron bond.