The original examples of lanthanide tetrad effect in solvent extraction: A new interpretation compatible with recent progress in REE geochemistry

摘要

Geochemical studies reporting lanthanide tetrad effects in natural samples often cite the original examples of tetrad effect depicted by logK(d)(org/aq) for Ln(III) in the two solvent-extraction systems (Peppard et al., 1969) and its theoretical interpretation (Jorgensen, 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 logK(d)(org/aq) data. The logK(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 Ln(3+) ions in aqueous and organic phases, DeltaE(3)/DeltaE(1), 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 (DeltaE(3)/DeltaE(1) = 0.23 +/- 0.02). The tetrad effects in logK(d)(org/aq) are experimental evidence for the thermochemical consequence of the nephelauxetic effect in Ln(III) complexes. Although Ln(3+) ions in condensed phases are atomic-like, it is rather surprising that logK(d)(org/aq) can easily be reproduced by the simple RSPET equation derived from the Slater-Condon-Racah theory for free Ln(3+) ions in vacuum. The effect of coordination change of Ln(3+) ion across the Ln(3+)(aq) series is not obvious in the series variations of logK(d)(org/aq), suggesting that the coordination changes in the two Ln(III) complex series in the extraction systems are significantly canceled out in DeltaG(r)degrees. The two sets of logK(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. [References: 35]