Structure and Energetics of the Samarium Diiodide−HMPA Complex in Tetrahydrofuran

The role of HMPA as a ligand for SmI 2 and the stoichiometry and energetics of formation for the SmI 2 −HMPA complex in THF were investigated employing UV−vis spectroscopy, isothermal titration calorimetry (ITC), and vapor pressure osmometry (VPO). The aggregation number for SmI 2 in THF was found to be 0.98 ± 0.09 over the entire concentration range studied (6.71−84.0 mM), indicating that SmI 2 is monomeric. The UV−vis data suggest that four HMPA ligands coordinate to SmI 2, and this was supported by ITC experiments. The combined results are consistent with [SmI 2 (HMPA) 4 ] being the reductant responsible for the unique reactivity exhibited by SmI 2 −HMPA cosolvent combinations.