Inner-sphere and outer-sphere complexes of yttrium(III), lanthanum(III), neodymium(III), terbium(III) and thulium(III) with halide ions in N,N-dimethylformamide

Abstract

The formation of chloro, bromo and iodo complexes of yttrium(III), and bromo and iodo complexes of lanthanum(III), neodymium(III), terbium(III) and thulium(III) has been studied by precise titration calorimetry in N,N-dimethylformamide (DMF) at 25 °C. The formation of [YCl]²⁺, [YCl₂]⁺, [YCl₃] and [YCl₄]^–, and [MBr]²⁺ and [MBr₂]⁺(M = Y, La, Nd, Tb, Tm) was revealed, and their formation constants, enthalpies and entropies were determined. Stabilities of these bromo complexes are smaller than those of the corresponding chloro complexes, which is expected for hard metal(III) ions. However, formation enthalpies of monobromo complexes of these metal ions, all showing positive values, are significantly smaller than those of the chloro complexes. Furthermore, the formation of iodo complexes of these metal(III) ions was evidently exothermic, though measured enthalpies were too small to determine reliable thermodynamic parameters. It is thus found that the formation enthalpies change in the sequence ΔH°(Cl) > ΔH°(Br) > ΔH°(I), which is unusual for hard metal(III) ions. This implies that, unlike the chloride ion, the bromide ion forms outer-sphere complexes with the lanthanide(III) and yttrium(III) ions in DMF. Evidence for either an inner- or outer-sphere complex was obtained from ⁸⁹Y NMR spectra for Y(CIO₄)₃, YCl₃ and YBr₃ DMF solutions at room temperature.