The exceptions to the Walsh rules: Linear and cyclic structures of EX2 (E = C, Si, Ge, Sn, Pb and X = O, S, Se)

The stable minima of the EX2 molecule (E = C, Si, Ge, Sn, Pb, and X = O, S, Se) with linear and cyclic structures were calculated at CCSD(T) and B3LYP levels. Most of dioxides favor linear structure following Walsh rules. However, lead dioxide is stabilized as cyclic structure. For sulfides, both cyclic SnS2 and PbS2 are found to be lower in energy while CS2 and SiS2 keep linear structure as most stable formula. As for selenides, more cyclic structures are located in lower energy than linear structures. The great difference between the linear and cyclic structures was mainly caused by relativistic effects, where greater size difference between valence s and p orbital in heavier atoms have lower tendency to form sp hybrid orbital. Finally the cyclic structure was analyzed in detailed according to NBO data and selected molecule orbital.