Studies on Electrostatic Effect and Tetrad Effect in some Mixed-ligand Lanthanide Complexes with Alizarine Red Sulphonate

Department of Chemistry, Dr. Harisingh Gour Vishwavidyalaya, Hagar-470 003 

Manuscript received 8 July 1986,  revised 14 October 1987, accepted 13 November 1987

The influence of electrostatic effect accompanying the formation of mixed-ligand complexes of the type [MAL], where M=La^III. Ce^llI, Pr^III, Nd^III, Sm^lII, 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 $K_{ML}^M$) and 1 : 1 : 1 ternary (log $K_{MAL}^{MA}$) complexes. The formation constants have been determined at 25° in aqueous medium by using the Irving - Rossotti approach at an ionic strength, I=0.2 (mol dm^-3, NaCIO₄). The ∆ log K (=log $K_{MAL}^{MA}$ — log $K_{ML}^M$) 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 bond and some stability correlations have also been studied.