C Al4 X (X=Si,Ge): Molecules with simultaneous planar tetracoordinate carbon, aluminum, and silicon/germanium

Abstract

The authors report the first theoretical study on the hexa-atomic molecules C Al4 X (XSi,Ge) at the B3LYP6-311++G (2d), MP26-311++G (2d), and CCSD (T) 6-311++G (3df) (single point) levels. Three low-lying isomers (within 2.0 kcalmol) can be formally viewed as constructed by one Al+ interacting with the planar C Al3 X- at the side Al-X bond (X-1), side Al-Al bond (X-2), and central C atom (X-3). The isomers X-1 and X-2 both have planar structures that include the planar tetracoordinate carbon, aluminum, and silicon/germanium, while the three-dimensional isomer X-3 has the pentacoordinate carbon. The planarity of X-1 and X-2 is ascribed to the ligand five-center two-electron bonding molecular orbital, similar to the orbital responsible for the planarity of C Al3 X- (XSi,Ge). Kinetically, the two planar structures X-1 and X-2 can be easily interconverted to each other via the intermediate X-3, indicative of their coexistence. Of particular interest, isomer X-1 represents the first example that simultaneously contains three types of planar centers in a single molecule, to the best of our knowledge. The three low-lying and structurally interesting isomers X-1, X-2, and X-3 await future experimental verification. The present results could enrich the planar chemistry. © 2007 American Institute of Physics.