The illustration of (a) methyl substitution and (b) ethyl substitution of TETC. IS, TS, and FS stand for initial state, transition state, and final state, respectively. In the ball and stick modelling, all the unlabeled dark color balls represent carbon atoms and all the unlabeled light color balls represent hydrogen atoms. The dark green balls, light green balls, and pink balls represent tin atoms, chloride atoms, and boron atoms, respectively. Only the hydrogen atoms in the reactive alkyl radical are displayed. The dash lines mean the nonbonding weak interactions between atoms.