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 CAlnBemHxq (n + m = 5, q = 0, +/- 1, x = q + m - 1) (1a-9a) by attaching H atoms onto the Be atoms in CAl4Be, CAl3Be2-, CAl2Be32-, and CAlBe43-. These ppC species are sigma and 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.