Deciphering Pyramidanes: A Quantum Chemical Topology Approach.

C[C₄H₄], the simplest compound of the [4]-pyramidane family, has so far eluded experimental characterization, although several of its analogs, E[C₄(SiMe₃)₄] in which the E apex atom is a tetrel group element, have been successfully prepared. The non-classical bonding mode of E, similar to that found in propellanes, has prompted a considerable number of theoretical studies to unravel the nature of the apex-base interaction. Here, we contribute to this knowledge by analyzing the electron localization function (ELF) and classical QTAIM descriptors; as well the statistical distribution of electrons in atomic regions by means of the so-called electron distribution functions (EDFs), calculation of multicenter indices (MCI) as aromaticity descriptors and by performing orbital invariant energy decompositions with the interacting quantum atoms (IQA) approach on a series of E[C₄(SiMe₃)₄] compounds. We find that the bonding evolves from covalent to electrostatic as E changes from C to Pb, with an anomaly when E=Si, which is shown to be the most charged moiety, compatible with an aromatic [C₄(SiMe₃)₄]^2- scaffold in the pyramidane base.