Tetrad effect in the solution stabilities of mixed ligand complexes of lanthanides

The occurrence of tetrad effect in the solution stabilities of mixed ligand complexes of lanthanides has been studied for the ternary systems of the type (Ln(III).A.L), where Ln(III)=La3+, Ce3+, Pr3+, Nd3+, Eu3+, Gd3+, Tb3+ or Dy3+' A=HEDTA, CDTA or DTPA; and L=IMDA. The formation constants logKsub(MAL)sup(MA), logKsub(ML)sup(M) and logKsub(ML)sup(ML) have been determined by potentiometric pH-titrations using Irving-Rossotti approach at 25degC and at ionic strength, I=0.2 (mol dm-3, NaClO4). The formation constants of the mixed ligand complexes follow the sequence: HEDTA>CDTA>DTPA, reflecting mainly the electrostatic effect. The ΔlogK values are negative. The stability data indicate the occurrence of tetrad effect at Gd(III). A differential plot method has been used to locate the occurrence of tetrad effect at the one-fourth and three-fourths filled 4f-shells also. The so-called inclined -W plots have been obtained on correlating the formation constants with the total orbital angular momentum quantum number, Lsub(Ln), of the lanthanides. The implications of periodicity in Lsub(Ln) have been discussed with reference to the inclined-W plots. (author). 33 refs., 5 figs