THERMODYNAMICS AND STRUCTURE OF THIOCYANATO COMPLEXES OF YTTRIUM(III) AND LANTHANIDE(III) IONS IN N,N-DIMETHYLFORMAMIDE

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-degrees-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. Y-89 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.