Planar tetracoordinate Si and Ge in π-aromatic (X = Si, Ge) cations

On the basis of the extensive density functional theory and wave function theory calculations performed in this work, we predict the existence of the perfectly planar triangle D3h X 3 Cu 3 + (X = Si, Ge) which are the global minima of the systems. The geometrical and electronic structures of X 3 Cu 3 + (X = Si, Ge) have been investigated at both B3LYP and MP2 methods with the 6-311 + G(3df) basis set. Interestingly, there are three planar tetracoordinate silicon and germanium atoms in D3h Si 3 Cu 3 + and Ge 3 Cu 3 + cationic species. NBO analyses for the series of clusters reveal that the Si–Si and Ge–Ge bonds are all the typical σ covalent bond. There are mainly electrostatic interactions between the bridge Cu and Si atoms in these planar cationic species. Nucleus independent chemical shifts (NICS) and electron localization function (ELF) analyses indicate that D3h Si 3 Cu 3 + and Ge 3 Cu 3 + are all π aromatic in nature. These novel cationic species turn out to be strongly thermodynamically favored in the gas phases and may be targeted in future experiments to expand the structural domain of planar tetracoordinate nonmetal chemistry.