Stabilization of beryllium-containing planar pentacoordinate carbon species through attaching hydrogen atoms

The diagonal relationship between beryllium and aluminum and the isoelectronic relationship between BeH unit and Al atom were utilized to design nine new planar and quasi-planar pentacoordinate carbon (ppC) species CAl_nBe_mH_x ^q (n + m = 5, q = 0, ±1, x = q + m − 1) (1a-9a) by attaching H atoms onto the Be atoms in CAl₄Be, CAl₃Be₂ ⁻, CAl₂Be₃ ²⁻, and CAlBe₄ ³⁻. These ppC species are σ and π double aromatic. In comparison with their parents, these H-attached molecules are more stable electronically, as can be reflected by the more favourable alternative negative-positive-negative charge-arranging pattern and the less dispersed peripheral orbitals. Remarkably, seven of these nine molecules are global energy minima, in which four of them are kinetically stable, including CAl₃Be₂H (2a), CAl₂Be₃H⁻ (4a), CAl₂Be₃H₂ (5a), and CAlBe₄H₄ ⁺ (9a). They are the promising target for the experimental realization of species with a ppC.