The coordination of a Lewis basic ligand containing X-H bonds (where X = O, N, etc.) to a redox-active metal in a higher oxidation state leads to a substantial heterolytic weakening of the coordinated ligand producing a stronger acid. However, coordination to the same metal in a lower oxidation state has a less profound impact on the acidity of the coordinated ligand. Conversely, coordination of ligands with X-H bonds to low-valent metals leads to a significant decrease of the ligand bond dissociation energy. This unique feature of ligand coordination to a redox active center is best exemplified by the work of Kovacs and colleagues shown in Scheme 1. Scheme 1. Thermochemical square for the dehydrogenation of I The pKa of water coordinated to the iron(II) complex I is 8 orders of magnitude less acidic than coordination to the iron(III) analogue II. However, coordination of water to I leads to a bond-weakening of the O-H of approximately 54 kcal/mol. There are a number of other examples of bond-weakening that demonstrate the impact of coordination to low-valent metals through coordination of protic ligands. An impressive example by Peters and coworkers established that protonation of Cp*2Co and related low-valent metallocenes leads to the formation of an incredibly weak C-H bond providing a unique approach for the development of strong proton-coupled electron transfer (PCET) donors and PCET mediators. While all of the aforementioned examples provide strategies to induce bond-weakening, the use of this approach in synthesis is only being fully realized, predominantly through the work of Knowles and coworkers. Over the past several years we have explored the mechanism of reactions that use the combination of samarium diiodide (SmI2) and water. This reagent system is unusual since it facilitates the reduction of substrates that have significantly more negative reduction potentials than the Sm(II)-water complex. Work in our group and that of Mayer provides compelling evidence that the reduction of many substrates by SmI2-water proceeds through proton-coupled electron transfer (PCET). While the use of Sm(II)-water in synthesis is well-documented, it can also be employed in other systems as exemplified by the elegant work of Nishibayashi demonstrating that the reagent system can be used to reduce nitrogen through molybdenum catalyzed fixation. Recently, we proposed that the key feature responsible for the unique reactivity of the Sm(II)-water complex is strong coor-dination that leads to significant weakening of the O-H bond of ligated water. If this supposition is correct, the stronger the coordination between a low-valent metal and ligand, the greater degree of bond weakening that should occur. The thermochemical cycle in Scheme 2 provides a thermodynamic argument in favor of this point demonstrating that the change in the bond dissociation free energy (BDFE) will be greater for ligands that have a strong interaction with a low-valent metal. Reagents based on Sm(II) provide a unique platform to test this supposition because a range of anionic ligands can be readily exchanged to tune the redox potential of the reagent and the large coordination sphere enables the coordination of a variety of protic ligands. Furthermore Lewis basic ligands such as glycols and amino alcohols that coordinate with varying affinities to Sm(II) are easily accessible. Herein we demonstrate that the appropriate choice of chelating protic Lewis base and Sm(II) reductant can be identified that combines complex stability and reactivity enabling the reduction of a range of substrates through PCET that cannot be reduced by electron transfer alone. Scheme 2. Thermochemical cycle for metal-ligand association and H-atom loss from the resulting complex. To initially examine the relationship between protic ligand affinity for Sm(II) and reactivity, a series of strongly reducing Sm(II) reductants in THF were examined including samarium dibromide (SmBr2), samarium dichloride (SmCl2), decamethyl samarocene (SmCp*2), and Sm(II) bis-trimethylsilyl amide (Sm(HMDS)2). Addition of 15 equivalents of ethylene glycol and ethanolamine with respect to [Sm(II)] was initiated for each reductant in a sealed round bottom flask under an Ar atmosphere and the evolution of H2 was monitored via a pressure sensor over ten minutes. Most combinations evolved H2 too rapidly to be useful, but among those examined, SmBr2 and ethanolamine provided the most stable combination of reagents. We have previously demonstrated that the steric bulk of an alcohol impacts the affinity for Sm(II). To determine whether the affinity of an ethanolamine for Sm(II) could be further modulated, a series of ligands including ethanolamine (EA), N-methylethanolamine (NMEA), and N,Ndimethylethanolamine (DMEA) were examined using UV-vis (Figure ). As predicted, EA had the highest affinity for SmBr2 in THF followed by NMEA and DMEA as measured by UV-vis spectroscopy. Although EA had the highest affinity for SmBr2 it was the least stable among the series. The combination of SmBr2-NMEA was the most promising since it had a high affinity for the metal and was stable within a reasonable timeframe for further reactivity studies. To assess the reactivity of the SmBr2-NMEA reagent system, a series of arenes were examined that form successively weaker C-H bonds upon formal hydrogen atom transfer (HAT) as shown in Figure . All arenes were reduced by SmBr2-NMEA in THF rapidly at room temperature. Trans-stilbene and naphthalene provided clean reductions while the reduction of biphenyl produced several products including cyclohexylbenzene. Given the moderate rate of reduction of 1methoxynaphthalene by SmBr2-NMEA it was identified as an ideal substrate for kinetic studies to examine the mechanism of arene reduction by SmBr2-NMEA. Stopped-flow spectrophotometry was used to determine the rate of the reaction and the rate order of each component by monitoring the decay of the characteristic absorption of Sm(II) at 540 nm. The results of these kinetics studies and a sample decay curve are shown in Figure . The results of the kinetic studies reveal that the reaction is approximately first-order in each component with a rate constant 4.9 M -2 s -1 for the reduction of 1-methxynaphthalene by SmBr2-NMEA at room temperature. Rate measurements using N-, O-deuterated NMEA provided a kinetic isotope of 1.2.