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

Geochemical studies reporting lanthanide tetrad effects in natural samples often cite the original examples of tetrad effect depicted by logK(d)(org/a q) for Ln(III) in the two solvent-extraction systems (Peppard et al., 1969) and its theoretical interpretation (Jorgensen, 1970; Nugent, 1970). An imp roved equation for the refined spin-pairing energy theory (RSPET), which ha s been developed in pursuing tetrad effects in geochemical studies, makes i t possible to examine more thoroughly the original examples of tetrad effec t by using the digitized logK(d)(org/aq) data. The logK(d)(org/aq) values a re found to be successfully described by the RSPET equation. Our analyses s how 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/a q) are experimental evidence for the thermochemical consequence of the neph elauxetic effect in Ln(III) complexes. Although Ln(3+) ions in condensed ph ases are atomic-like, it is rather surprising that logK(d)(org/aq) can easi ly be reproduced by the simple RSPET equation derived from the Slater-Condo n-Racah theory for free Ln(3+) ions in vacuum. The effect of coordination c hange of Ln(3+) ion across the Ln(3+)(aq) series is not obvious in the seri es variations of logK(d)(org/aq), suggesting that the coordination changes in the two Ln(III) complex series in the extraction systems are significant ly canceled out in DeltaG(r)degrees. The two sets of logK(d)(org/aq) are an alogous to the Masuda-Coryell plot in REE geochemistry with respect to thei r coordinates. This gives an insight into the fact that the Masuda-Coryell plot also depicts tetrad effects in the REE patterns for geochemical sample s.