THERMODYNAMICS OF CHLORO COMPLEXATION OF LANTHANIDE(III) IONS IN N,N-DIMETHYLFORMAMIDE

The chloro complexation of a series of Ln(III) ions (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) has been studied by calorimetry in N,N-dimethylformamide (DMF) containing 0.2 mol dm-3 (C2H5)4NClO4 as a constant ionic medium at 25-degrees-C. The formation of [LnCl]2+, [LnCl2]+, [LnCl3], and [LnCl4]- complexes is revealed for all the metal systems except that of La(III). The [LaCl4-] complex hardly forms. The large and positive enthalpy and entropy values show the favorable formation of inner-sphere chloro complexes for all the lanthanide(III) systems. The DELTA-H1-degrees and DELTA-S1-degrees values decrease with the atomic number Z for La(III), Ce(III), Pr(III), and Nd(III), while the values gradually increase with Z for Sm(III)-Er(III). With respect to Tm(III), Yb(III), and Lu(III), the DELTA-H1-degrees and DELTA-S1-degrees values are particularly large and similar. The variation of DELTA-H1-degrees and DELTA-S1-degrees values along Z suggests that coordination energies and geometries of Ln3+ solvates vary significantly with decreasing ionic radii. It is proposed that an equilibrium [Ln(DMF)8]3+ + DMF = [Ln(DMF)9]3+ is established, which shifts to the right in the order La > Ce > Pr > Nd.