G3-Global and -Local Minima of XC2H4 + Structural Isomers where X = Li to Br

B3LYP/6-311+G** and G3 levels of theory reveal that cyclic and 1-substituted isomers are either global or local minima for XC2H4 + structural isomers when X is CH3 or an element of groups V, VI and VII. π-bridged structures are global minima when X is an element of groups I, II and III, SiH3 and GeH3. The large electrophilicity of C+ in substituted ethyl cations and the position of X in the periodic table are the main factors that determine the structure of the global minima