Hydrogen, molecular, interaction with hydrocarbon species

The cleavage of the C—H bond by direct participation of a transition metal ion proceeds via an oxidative addition mechanism or an electrophilic substitution mechanism. Metals in low oxidation states undergo oxidative addition while high oxidation state metals take part in electrophilic substitutions. Another function of the metal complex in these reactions consists of abstracting an electron or a hydrogen atom from the hydrocarbon, RH. The RH radical ions or R radicals which are formed then interact with other species, such as molecular oxygen which is present in the solution or in one of the ligands of the metal complex (21). 

Evidence from the kinetics of both olefin 27, 31) and acetylene 71) hydrogenation and from the exchange of cycloalkanes with deuterium 12) (in which 7r-allylic intermediates are formed) has shown almost unequivocally that molecular hydrogen can interact with adsorbed hydrocarbon species. When this is the case, addition takes place from above the axis of unsaturation. The importance of molecular hydrogen interaction, as a general phenomenon, has yet to be determined. However, the evidence is sufficiently strong to compel the consideration of steps such as... 

---