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 hyparene'' analogues CB3E2q (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 CB3E2q (q = +/- 1) species were involved in three delocalized sigma orbitals and a delocalized pi orbital, so the carbon atom obeys the octet rule. The NICS studies indicated that these ppC structures are sigma and pi 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.