Multiconfigurational and density functional theories are adopted to study the potential energy surface along the possible reaction mechanisms of I-pyrazoline on the Si(100) 2 x I surface. According to the computed potential energy surfaces, the Si-N single bonded product is initially formed without a reaction barrier due to the strong Lewis base of the nitrogen lone pair electrons. With a mild internal activation barrier, the most stable [2 + 2] cycloaddition product is formed. The surface Si-Si sigma bond broken product was found to be stable, which requires 4.4 kcal/mol of overall activation barrier, making it kinetically quite accessible. The surface isomerization reaction product, which has two Si=N double bonds as a result of breakage of both the surface Si-Si sigma bond and the N-N sigma bond, is found to be thermodynamically as well as kinetically stable, suggesting a possible route to multiple interfacial Si=X bond.