Interactions of Bis(2,4,4‐trimethylpentyl)dithiophosphinate with NdIII and CmIII in a Homogeneous Medium: A Comparative Study of Thermodynamics and Coordination Modes

Complexation of Nd^III and Cm^III with purified Cyanex301 (ammonium bis(2,4,4‐trimethylpentyl)dithiophosphinate, denoted as HL) was studied in 1 % v/v water/ethanol under identical conditions by spectrophotometry and microcalorimetry. For Nd^III, three successive complexes, NdL²⁺, NdL₂⁺, and NdL₃, formed in the solution. In contrast, four complexes, CmL²⁺, CmL₂⁺, CmL₃, and CmL₄^? formed during the titration with Cm. Fluorescence lifetime measurements provided additional insight into the complexation of Cm^III with Cyanex301. The stepwise stability constants for the CmL_j^(3?j)+ (j=1–3) complexes are about one order of magnitude higher than the corresponding NdL_j^(3?j)+ complexes. The enthalpies of complexation are endothermic for both Nd^III and Cm^III, suggesting that the energy required for desolvation exceeds the energy gained from the cation/ligand combination. Specifically, the enthalpy of complexation for CmL²⁺ is 3.5 kJ mol^?1 less endothermic than that of NdL²⁺, implying stronger covalent interaction in CmL²⁺ than NdL²⁺. However, the enthalpies of complexation for CmL₂⁺ and NdL₂⁺ are nearly identical, and the enthalpy of complexation for CmL₃(aq) becomes more endothermic than that for NdL₃(aq). The observations suggest that, in the ethanol/water media, the overall energetics of the Cm^III/Nd^III complexation with Cyanex301 could depend on a number of factors, including the extent of covalency, the degree of desolvation, and the coordination modes.