https://doi.org/10.1351/goldbook.S05772
This is analogous to the process of ligand insertion in coordination chemistry. $$\ce{H^{\ast} + C2H4(g) -> ^{\ast}C2H5}$$ This reaction might also be imagined to proceed by adsorption of $\ce{C2H4}$ followed by ligand migration (an associative surface reaction).