@misc{oai:nagoya.repo.nii.ac.jp:00018233,
author = {Kawabe, Iwao},
month = {Jun},
note = {The logarithmic formation constants (logK) for aqueous lanthanide(III)-EDTA complexes show a Gd-break and subtle tetrad effect, but the ΔSr and ΔHr data indicate step-like changes in the middle Ln. UV-Vis spectra of Eu3+ for Eu-EDTA solution suggest the hydration change occurring across the middle Ln-EDTA series. This is supported by the recent studies of luminescence kinetics on the inner-sphere waters of Eu-EDTA and Tb-EDTA, and by the lanthanide-induced 17O NMR shifts of Ln-EDTA. Such spectroscopic studies allow to re-examine the long-lasting controversy of the Gd-break and tetrad effect of logK(Ln-EDTA). Here is proposed a thermodynamic model to elucidate it from the viewpoints: i) hydration changes of Ln-EDTA and light Ln3+ (aq) series, ii) the nephelauxetic effect due to the coordination change of Ln3+ of Ln-EDTA, and iii) the improved equation of Jφrgensen theory applicable to ΔHr and ΔGr. This model satisfies the spectroscopic constraints on the hydration states of Ln-EDTA series: Three and two water molecules exist in the inner spheres of light Ln(La~Nd)-EDTA and heavy Ln(Dy~Lu)-EDTA series, respectively. Each middle Ln(Sm~Tb)-EDTA is a mixture of the two hydrate species, and their abundances are given by the equilibrium of hydration change reaction: [Ln ⋅ EDTA⋅ (H2O)3 ]− (aq ) = [Ln ⋅ EDTA⋅ (H2O)2 ]− (aq ) + H2O(l ). The ΔHr, ΔSr and ΔGr data for Ln-EDTA formation, when corrected for hydration changes in Ln-EDTA and light Ln3+ (aq) series, can be fitted by the improved equation of Jφrgensen theory, and the thermodynamic parameters for the formation of two Ln-EDTA series from octahydrate Ln3+ (aq) have been evaluated. The hydration change of middle Ln-EDTA series explains the apparent Gd-break of logK(Ln-EDTA). The nephelauxetic effect by the change in coordination number for Ln3+ (ΔCN) is evident in ΔH values for the three reactions: (a) [Ln ⋅ EDTA⋅ (H2O)3 ]− (aq ) formation from Ln(oct, aq ) 3+ (ΔCN=+1), (b) [Ln ⋅ EDTA⋅ (H2O)2 ]− (aq ) formation from Ln(oct, aq) 3+ (ΔCN=0), and (c) the hydration change reaction of Ln-EDTA given as (b)-(a) (ΔCN=−1). The relative Racah E1 parameter values for Nd3+, ΔE1(Nd3+), from the tetrad effects of ΔH for (a), (b) and (c), are +(39±14) cm-1, −(19±7) cm-1, and −(58±18) cm-1, respectively. The ΔHr values for (a) with ΔCN=+1 show a large convex tetrad effect, and the ΔSr values also exhibit a similar tetrad effect. Therefore, their tetrad effects are significantly cancelled in ΔGr. This is a reason why the subtle tetrad-like variation of logK(Ln-EDTA) is asymmetrical between light and heavy Ln, along with the hydration change effects of middle Ln-EDTA and light Ln3+ (aq) series cancelled partly within the light Ln.},
title = {A puzzle of Gd-break and tetrad effect of aqueous lanthanide(III)-EDTA complex formation: Different Racah parameters between two lanthanide-EDTA complex series with distinct hydration states},
year = {2014}
}