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) compl exes were identified at low temperature with a large excess of L. Conductom etry measurements showed for U(III) a large increase in the conductivity wh en increasing the molar ratio L:U. The complex UL2 was found to be a 1: 1 e lectrolyte and the species UI2+ was more reactive toward L in comparison wi th UI3. For Ce(IH) and Nd(IH), MI2+ and MI3 present about the same affinity for L. The stability of the complexes is limited, and U(III) possesses a s lightly higher affinity for bipy than the trivalent lanthanides. Interestin gly, a preference for the formation of ML2 Complex was shown for all the st udied M(III) ions. The driving force for complex formation was always the e nthalpy, and, surprisingly for a bidendate ligand (bipy), no favorable entr opy contribution to complex formation was observed. The X-ray crystal struc tures of [CeI3(bipy)(2)(py)](4)(.)5py-bipy and UI3(bipy)(2)(py)(.)2py were determined. The structures of the molecules MI3(biPY)(2)(Py) are almost ide ntical for U and Ce. The mean M(III)-N(bipy) bond distances are equal to 2. 67(3) Angstrom for Ce(IH) and 2.65(4) Angstrom for U(III). The slightly sma ller M(IH)-N(bipy) distances observed for U(III) would reflect a slightly m ore important covalent character of the U(III)-N(bipy) bonds, in agreement with the slightly better affinity of U(III) than Ce(III) or Nd(HI) toward b ipy observed in solution and with the fact, that the enthalpy is the drivin g force for complex formation.