Mechanistic Insights into H–H Bond Cleavage and [1 + 2] Cycloaddition of Ethylene Mediated by Triplet Pnictinidenes

undergoes [1 + 2] cycloaddition with ethylene through a triplet transition state, ultimately producing singlet three-membered heterocycles after intersystem crossing. Activation strain model (ASM) analyses reveal that the atomic radius of the central G15 atom provides a consistent explanation for the origins of the activation barriers in both H-H bond cleavage and [1 + 2] cycloaddition reactions. This study offers qualitative rationalization based on the activation barrier and assesses the feasibility of the proposed experimental observations.