The methodd eveloped and introduced here enables for the first time (to the authors' knowledge), aq uantitative assessment of the interaction of SmI 2 with substrates prior to the electron transfer stage. As ap roof of concept, equilibrium constants for some model substrates including carbonyl compounds and aromatic nucleia re reported here. In addition, the first equilibrium constants with some common ligands were also determined. The equilibrium constants range from approximately 0.07 m À1 for diisopropyl ketone to 2500 m À1 for hexamethylphosphoramide (HMPA). It is shown that the data acquiredb yt his method,w hich is based on the concept of shift reagents, can shed light on the most intimate detailso ft he reaction mechanism, and this methodi sauseful toolf or planning as ynthetic process.[a] Aand Ba re computational methods.Please see text.