Addition of HMPA to [Sm{N(SiMe3)(2)}(2)] produces a less reactive reductant in contrast to addition of HMPA to Sml(2). While the [Sm{N(SiMe3)(2)}(2)]-HMPA combination results in a more powerful reductant based on the redox potential, the observed decrease in reactivity is attributed to steric hindrance caused by the nonlabile ligand -N(SiMe3)(2) and HMPA around the Sm metal. The importance of ligand displacement (exchange) in Sm(II)-HMPA-based reactions and insight into the mechanism of [Sm{N(SiMe3)(2)}(2)]-HMPA and Sml(2)-HMPA reductions are presented.