The stability constants, β1, of each monochloride complex of Ln(III) (Ln=Nd or Tm) have been determined in the mixed system of dimethyl sulfoxide (DMSO)
and water with 1.0 mol·dm−3 ionic strength using a solvent extraction technique. The values of β1 of Ln(III) decrease to about 0.2 mole fraction of DMSO (Xs) in the mixed solvent system and then increase withXs (>about 0.2). However, the variation mode of β1 of Nd(III) withXs somewhat differs from that of Tm(III). Calculation of Ln3+−Cl− distance using a Born-type equation of the Gibbs' free energy derived from the β1 evealed the followings: (1) For Tm3+ with coordination number 8, the estimated distance between Tm3+ and Cl− (dTm-Cl) increases linearly withXs in 0.00≤Xs≤0.17. This means an enlargement of the primary solvation sphere size of Tm3+ withXs. On the other hand, thedTm-Cl shows a decrease withXs in 0.17<Xs<0.28. (2) The estimateddNd-Cl increases linearly withXs in 0.00≤Xs<0.06 and 0.06<Xs≤0.17, but their slopes are different. The larger slope againstXs in 0.06<Xs≤0.17 is attributable to a lowering of the β1 by a coordination of ClO4− into the secondary solvation sphere of Nd3+ and/or by an increase in the solvation number of the primary solvation sphere of Nd3+.