Effect of HMPA on the Ground and Excited State Reactivity of SmI ₂

Samarium Diiodide (SmI 2 ) is a privileged reducing agent in organic synthesis. Tunability of reactivity and selectivity of SmI 2 through additives made this reagent an attractive choice for organic chemists. Present investigation showed contrasting effects of hexamethylphosphoramide (HMPA) on the ground and excited states of reactions of SmI 2 with haloalkanes. While HMPA enhances the rate of reduction of haloalkanes in the ground state, a deleterious impact was found for excited‐state reactions. Detailed investigations with kinetics, element effect, known BDE of CH bonds, and computed CX bond distance indicated that while ground state reactivity is governed by stability of radicals and weakening of CX bond due to HMPA and SmI 2 coordination, excited state reactions are predominantly governed by inner sphere nature of electron transfer. The critical role of HMPA in facilitating the reduction of haloalkanes was confirmed using density functional theory‐based calculations.