Studies on electrostatic effect and tetrad effect in some mixed-ligand lanthanide complexes with alizarine red sulphonate

The influence of electrostatic effect accompanying the formation of mixed-ligand complexes of the type (MAL), where M=La(III), Ce(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), or Dy(III); A=(primary ligand) CDTA or DTPA; L=(secondary ligand) alizarine red sulphonate (ARS), has been studied by comparing the formation constants of 1:1 binary (log Ksub(ML)sup(M) and 1:1:1 ternary (log Ksub(MAL)sup( MA)) complexes. The formation constants have been determined at 25deg in aqueous medi um by using the Irving- Rossotti approach at an ionic strength, I=0.2 (mol dm-2, NaClO4). The Δ log K (=log Ksub(MAL)sup(MA) -log Ksub(ML)sup(M) values have been found to be more negative with DTPA than with CDTA due to greater electrostatic repulsion with the former. The occurrence of tetrad effect has been discussed. The nature of M-L bond and some stability correlations have also been studied. (author). 3 figs., 1 table, 16 refs