Steric solvent effect on formation thermodynamics and structure of halogeno complexes of lanthanide(III) ions in N,N-dimethylacetamide

The bromo complexation of a series of LnIII ions (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) has been studied by precise titration calorimetry in N,N-dimethylacetamide (DMA) containing 0.2 mol dm–3(C2H5)4NClO4 as a constant ionic medium at 298 K. The formation of mono-, di- and tri-bromo complexes was established and their formation constants and enthalpies and entropies of formation were determined. With the monobromo complex, both the enthalpy and entropy values are positive and large and exhibit strong dependence on the metal ion. This result in DMA differs markedly from that in N,N-dimethylformamide (DMF). The difference is explained in terms of the formation of an inner-sphere complex in DMA, whilst in DMF a solvent-separated ion pair, or an outer-sphere complex, is formed. The coordination structure of the metal complexes has been evidenced by NMR for yttrium(III) and lanthanum(III) and by EXAFS for thulium(III). It is thus suggested that there is steric hindrance of the solvation of lanthanide(III) ions in DMA. The chloro complexation of LaIII and YIII in DMA was also examined for comparison.