Abstract
Experimental dG, dH and dS data for Ln(III) chelate complex formations appear interesting from the lanthanide tetrad effect. We report the series changes of dG, dH and dS for Ln-EDTA and Ln-DTPA formations examined by using Jorgensen-Kawabe equation for lanthanide tetrad effect. Ln-DTPA comprises an isostructural series with Ln(III) coordination of CN=9 across Ln series. The dH and dS for Ln-DTPA formation, when corrected for the hydration change of light Ln3+(aq) relative to the heavy Ln3+(aq), exhibit similar convex tetrad effects, but such tetrad effects are cancelled in dG. In Ln-EDTA series, light Ln and heavy Ln members have respectively CN=9 and 8, and a hydration change is occurring in the middle of Ln-EDTA series. dG, dH and dS for the formations of Ln-EDTA(CN=9) and Ln-EDTA(CN=8) from octahydrate Ln3+(aq) have been evaluated from the experimental data and a simple thermodynamic model. dH and dS for the Ln-EDTA(CN=9) formation show convex tetrad effects, but they are cancelled in dG almost totally, like the case of Ln-DTPA.
