Harpoon reactions are a type of chemical reaction between two substances: one (generally a metal) prone to form a cation, and the other (generally a halogen) prone to form an anion.
Their main feature is that these reactions, unlike most reactions, have steric factors greater than unity, that is, they take place faster than predicted by collision theory. This is explained by the fact that the colliding particles have greater cross sections than the pure geometrical ones calculated from their radii, because when the particles are close enough, an electron "jumps" (therefore the name) from one of the particles to the other one, forming an anion and a cation which subsequently attract each other. Harpoon reactions usually take place in the gas phase, but they are also possible in condensed media.
The predicted rate constant can be improved by using a better estimation of the steric factor. A rough approximation is that the largest separation Rx at which charge transfer can take place on energetic grounds, can be estimated from the solution of the following equation that determines the largest distance at which the Coulombic attraction between the two oppositely charged ions is sufficient to provide the energy ΔE0
With