On the variation of the distances of Nd3+-Cl- and Tm3+-Cl- in mixed system of dimethyl sulfoxide and water

The stability constants, beta(1), 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 beta(1) of Ln(III) decrease to about 0.2 mole fraction of DMSO (X-s) in the mixed solvent system and then increase with X-s (>about 0.2). However, the variation mode of beta(1) of Nd(III) with X-s somewhat differs from that of Tm(III). Calculation of Ln(3+)-Cl- distance using a Born-type equation of the Gibbs' free energy derived from the beta(1) revealed the followings: (1) For Tm3+ with coordination number 8, the estimated distance between Tm3+ and Cl- (d(Tm-Cl)) increases linearly with X-s in 0.00 less than or equal to X-s less than or equal to 0.17. This means an enlargement of the primary solvation sphere size of Tm3+ with X-s. On the other hand, the d(Tm-Cl) shows a decrease with X-s in 0.17<X-s<0.28. (2) The estimated d(Nd-Cl) increases linearly with X-s in 0.00 less than or equal to X-s<0.06 and 0.06<X-s less than or equal to 0.17, but their slopes are different. The larger slope against X-s in 0.06<X-s less than or equal to 0.17 is attributable to a lowering of the beta(1(Nd)) 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+.