Complexation Studies of Iodides of Trivalent Uranium and Lanthanides (Ce and Nd) with 2,2‘-Bipyridine in Anhydrous Pyridine Solutions

In anhydrous pyridine solution at 294 K, U(III) and Ce(III) triiodides were found to form both 1:1 (ML) and 1:2 (ML2) complexes with bipyridine (bipy = L) while Nd(III) triodide formed only a 1:2 complex. The 1:3 (ML3) complexes were identified at low temperature with a large excess of L. Conductometry measurements showed for U(III) a large increase in the conductivity when increasing the molar ratio L:U. The complex UL2 was found to be a 1:1 electrolyte and the species UI2+ was more reactive toward L in comparison with UI3. For Ce(III) and Nd(III), MI2+ and MI3 present about the same affinity for L. The stability of the complexes is limited, and U(III) possesses a slightly higher affinity for bipy than the trivalent lanthanides. Interestingly, a preference for the formation of ML2 complex was shown for all the studied M(III) ions. The driving force for complex formation was always the enthalpy, and, surprisingly for a bidendate ligand (bipy), no favorable entropy contribution to complex formation was obs...