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X2Ge=Sn: (X =H,Me,F,Cl,Br,Ph,Ar…) are new species of chemistry.The cycloaddition reaction of X2Ge=Sn: is a new study field of stannylene chemistry.To explore the rules of cycloaddition reaction between X2Ge=Sn: and the symmetric π-bonded compounds,the cycloaddition reactions of Me2Ge=Sn: and ethylene were selected as model reactions in this paper,and the mechanism was investigated for the first time here using the MP2 theory together with the 6-311++G** basis set for C,H and Ge atoms and the LanL2dzbasis set for Sn atoms.From the potential energy profile,it could be predicted that the reaction has one dominant reaction channel.The reaction rule present is that the 5p unoccupied orbital of Sn in Me2Ge=Sn: and the π orbital of ethylene form a π → p donor-acceptor bond,resulting in an intermediate which,due to its instability,makes itself isomerize into a four-membered Ge-heterocyclic ring stylene.Because the 5p unoccupied orbital of Sn atom in the four-membered Ge-heterocyclic ring stannylene and the π orbital of ethylene form a π→ p donor-acceptor bond,the four-membered Ge-heterocyclic ring stannylene further combines with ethylene to get another intermediate.Because the Sn atom in this intermediate exhibits sp3 hybridization after transition state,the intermediate isomerizes to a Ge-heterocyclic spiro-Sn-heterocyclic ring compound.The research result indicates the laws of cycloaddition reaction between X2Ge=Sn: and the symmetric π-bonded compounds.This study opens up a new research field for stannylene chemistry.