Thermodynamics and structure of thiocyanato complexes of yttrium(III) and lanthanide(III) ions in N,N-dimethylformamide

Abstract

The formation of thiocyanato complexes of yttrium(III) and lanthanide(III)(Ln = La, Nd, Tb, Ho, Tm, Yb) ions 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 °C. The data were well explained in terms of formation of a series of mononuclear [Ln(NCS)_n]^(3–n)+(n= 1–3) complexes, and their formation constants, reaction enthalpies and entropies were determined. ⁸⁹Y NMR and Raman spectra suggest that the metal ions form inner-sphere complexes with thiocyanate ions in DMF. Furthermore, with Yb^III and Ln^III, the Raman band for the CS stretching vibration of SCN^– shows an abnormal shift and broadening. Although less marked, the same effect was observed with La^III, Ce^III and Tm^III. This was explained in terms of establishment of a solvation equilibrium with the [Ln(NCS)]²⁺ complex, i.e. the complex consists of two distinct species which differ in the number of coordinating solvent molecules.