Proton magnetic resonance study of ligand exchange on tetrakis (hexamethylphosphoramide) dioxouranium(VI)

1H NMR studies show the rate law for hexamethylphosphoramide (HMPA) exchange on UO2(HMPA)42+ to be rate = 4(k1 + k2[HMPA])[UO2(HNPA)42+] in CD2Cl2 diluent, where k1(273 K) = 12.5 +- 1.1 s-1, ΔH2+ = 14.0 +- 3.0 kJ mol-1, and ΔS2+ = -172 +- 11 J K-1 mol-1 and k2(273 K) = 173 +- 15 mol-1 dm3 s-1, ΔH2+ = 22.2 +- 3.0 kJ mol-1, and ΔS2+ = -120 +- 9 J K-1 mol-1. This two-term rate law contrasts with the first-order rate law observed for the exchange of oxygen donor ligands L in some UO2L52+ species, and the origins of the two different rate laws are discussed in terms of dissociative and associative exchange mechanisms