The measured value of E°(SmI 2 + −HMPA − SmI 2 −HMPA) was used in combination with literature values for the E°of primary radicals to estimate the bimolecular rate constant for an outer-sphere electron transfer (ET). Comparison with the experimentally measured rate constant is consistent with an outer-sphere ET.Samarium diiodide (SmI 2 ) is one of the most important reducing reagents utilized by organic chemists. It is useful in functional group reductions, the coupling of halides with π bonds, and the coupling of two π bonds. 1 The most important feature of SmI 2 is its ability to promote one-pot, sequential reactions including both one-and two-electron processes. 2 The success of many SmI 2 -mediated reactions is often dependent on the addition of a ligating cosolvent such as HMPA, 3 which accelerates reactions of SmI 2 . 4 In addition to accelerating reductions by SmI 2 , HMPA also enhances the stereochemical outcome and diastereoselectivity of many reactions. 3,5 Although the combination of SmI 2 -HMPA in THF produces an extremely versatile reductant, there is a paucity of detail on its interaction with organic substrates.Recent results in our laboratory suggest that SmI 2 (HMPA) 4 is the species responsible for the unique reactivity of the SmI 2 -HMPA combination in THF. 6 Careful examination of the recently published X-ray crystal structure of the SmI 2 -(HMPA) 4 reductant shows that the complex is sterically crowded. 7 There are two potential ways that a reducible functional group (alkyl halide, carbonyl, or radical) can be reduced by the sterically crowded reductant (Scheme 1). The first would involve displacement of an SmI 2 (HMPA) 4 ligand by substrate which could lead to an inner-sphere electron transfer (ET). The second potential mechanistic pathway would have the ligands remain intact and the ET take place through an outer-sphere process. Since radicals are formed during numerous reductive coupling processes mediated by SmI 2 , we carried out the experiments described below to determine the mode of electron transfer from SmI 2 (HMPA) 4 to primary radicals.One of the most important values that needs to be defined in order to determine the mode of electron transfer is the standard potential of the SmI 2 (HMPA) 4 -SmI 2 + -(HMPA) 4 redox couple in THF. We recently utilized linear sweep voltammetry (LSV) to examine the influence of HMPA concentration on the reducing power of SmI 2 . 8 While these experiments showed unequivocally that HMPA increases the reducing power of SmI 2 , the reported values were not standard potentials. Figure 1 contains the cyclic voltammogram of SmI 2 and SmI 2 (HMPA) 4 in THF. The CV of SmI 2 containing HMPA did not change with the addition of more than 4 equiv of HMPA. Both voltammograms contained in Figure 1 are quasireversible, so we employed the model recently described by Skrydstrup for the SmI 2 + -SmI 2 redox (1) Curran, D. P.; Fevig, T. L.; Jasperse, C. P.; Totleben, M. J. Synlett 1992, 943-961.(2) Molander, G. A.; Harris, C. R.