CB3E2q(q=+-1) a family of hyparene'' analogues with a planar pentacoordinate carbon

      A CB3 moiety extracted from the building units of milestone "hyparenes" (families of species with a planar pentacoordinate carbon (ppC)) was found to be a more basic building block, which can be employed to design a family of "hyparenes" analogues CB₃E₂^q (q = ±1) also with a ppC. The majority of main group elements can feasibly serve as the E atom. Despite the number of valence electrons, the ppC atoms in the CB₃E₂^q (q = ±1) species were involved in three delocalized s orbitals and a delocalized p orbital, so the carbon atom obeys the octet rule. The NICS studies indicated that these ppC structures are s and p double aromatic. Given that most of them are less favourable in energy than their boron-centered isomers, it is remarkable that the global minimum of CB₃Mg₂^- adopts the ppC arrangement. Such a ppC structure is also kinetically stable. Compared to previously reported anionic ppC global minima, CB₃Mg₂^- does not contain hyper toxic beryllium and thus is much more attractive to our experimental colleagues for realizing the ppC species using negative ion photoelectron detachment spectroscopy.