Compared with the homolytic fission of the H-H bond, heterolytic cleavage of H2 is more likely to appear. Both the synergetic mechanism and stepwise mechanism can be used to demonstrate the process. An important example is the heterolytic fission of H-H bond with the protonation of a base (Figure ). In figure , H2 are likely to bind at a site transverse to bridging amine bases for instance and a proton moves to a basic site, which is represented as pink "B" atom. In the complexes, the amine bases are coordinated into the structure of the complexes. Nickel, iron, and cobalt diphosphine complexes can act as proton relays, which perform several functions for the activation and formation of hydrogen molecule: