Pentaatomic planar tetracoordinate carbon molecules [XCAl3]q [(X,q) = (B,−2), (C,−1), (N,0)] with C–X multiple bonding

Among the fascinating planar tetracoordinate carbon (ptC) species, pentaatomic molecules belong to the smallest class, well-known as “pptC”. It has been generally accepted that the planarity of pptC structure is realized via the “delocalization” of the pz lone pair at the central carbon and the ligand–ligand bonding interaction. Although “localization” is as key driving force in organic chemistry as “delocalization”, the “localization” concept has not been applied to the design of pptC molecules, to the best of our knowledge. In this paper, we apply the “localization” strategy to design computationally a series of new pptC. It is shown that the central carbon atom and one “electronegative” ligand atom X (compared to the Al ligand) effectively form a highly localized C–X multiple bond, converting the lone pair at the central carbon to a two-center two-electron π-bond. At the aug-cc-pVTZ-B3LYP, MP2 and CCSD(T) levels, the designed 18-valence-electron pptC species [XCAl3]q; [(X,q) = (B,−2), (C,−1), (N,0)] are found to each possess a stable ptC structure bearing a C–X double bond, indicated by the structural, molecular orbital, Wiberg bonding, potential energy surface and Born–Oppenheimer molecular dynamics (BOMD) analysis. Moreover, our OVGF calculations showed that the presently disclosed (yet previously unconsidered) pptC structure of [C2Al3]− could well account for the observed photoelectron spectrum (previously only ascribed to a close-energy fan-like structure). Therefore, [C2Al3]− could be the first pptC that bears the highly localized C–X double bond that has been experimentally generated. Notably, the pptC structure is the respective global minimum point for [BCAl3]2− and [NCAl3], and the counterion(s) would further stabilize [BCAl3]2− and [C2Al3]−. Thus, these newly designed pptC species with interesting bonding structure should be viable for future experimental characterization. The presently applied “localization” approach complements well the previous “delocalization” one, indicating that the general “localization vs. delocalization” concept in organic chemistry can be effectively transplanted to exotic pptC chemistry.