ABSTRACT

Geochmeical studies reporting lanthanide tetrad effects in natural samples often cite the original examples of tetrad effect depicted by log K_d(org/aq) for Ln(III) in the two solvent-extraction systems (Peppard et al., 1969) and its theoretical interpretation (Jørgensen, 1970; Nugent, 1970). An improved equation for the refined spin-pairing energy theory (RSPET), which has been developed in pursuing tetrad effects in geochemical studies, makes it possible to examine more thoroughly the original examples of tetrad effect by using the digitized log K_d(org/aq) data. The log K_d(org/aq) values are found to be successfully described by the RSPET equation. Our analyses show that the ratios of relative differences in Racah parameters for Ln³⁺ ions in aqueous and organic phases, ΔE³/ΔE¹, are 0.29 ± 0.07 and 0.25 ± 0.06 for the two extraction systems. They are comparable with the spectroscopic observations of nephelauxetic effect in Nd(III) compounds (ΔE³/ΔE¹ = 0.23 ± 0.02). The tetrad effects in log K_d(org/aq) are experimental evidence for the thermochemical consequence of the nephelauxetic effect in Ln(III) complexes. Although Ln³⁺ ions in condensed phases are atomic-like, it is rather surprising that log K_d(org/aq) can easily be reproduced by the simple RSPET equation derived from the Slater-Condon-Racah theory for free Ln³⁺ ions in vacuum. The effect of coordination change of Ln³⁺ ion across the Ln³⁺(aq) series is not obvious in the series variations of log K_d(org/aq), suggesting that the coordination changes in the two Ln(III) complex series in the extraction systems are significantly canceled out in ΔG_r°. The two sets of log K_d(org/aq) are analogous to the Masuda-Coryell plot in REE geochemistry with respect to their coordinates. This gives an insight into the fact that the Masuda-Coryell plot also depicts tetrad effects in the REE patterns for geochemical samples.