The isomeric structures and energetics of two Sn, Pb-containing penta-atomic planar tetracoordinate carbon molecules [CAl₃X] and [CAl₃X]^－ (X＝Sn, Pb) (with 17 and 18 valence electrons, respectively) were studied by using the B3LYP method combined with the mixed basis set 6-311＋G(d)&LANl2dz for the first time. The results were compared with the previously reported [CAl₃X] and [CAl₃X]^－ (X＝Si, Ge). It was shown that the isomer int1 with the planar tetracoordinate carbon structure was the global minimum point. Relative to the 17e system [CAl₃X], achieving an additional electron to form the 18e system [CAl₃X]^－ int1 becomes thermodynamically more stable. The present work would enrich the penta-atomic planar tetracoordinate carbon chemistry.

Key words: [CAl₃X], [CAl₃X]^－, penta-atomic planar tetracoordinate carbon, isomer, thermodynamic stability