Hydration change of aqueous lanthanide ions and tetrad effects in lanthanide(III)-carbonate complexation

Abstract

Lanthanide(III)-carbonate complexes, LnCO3+(aq) and Ln(CO3)2-(aq), are the principal Ln(III) species in seawater. Their logarithmic stability constants, logβ1(Ln(CO3)+) and logβ2(Ln(CO3)2-) defined by total carbonate ion concentration, are known to show 'irregular' variations across the series. The irregularities are explained by the hydration change of light Ln3+(aq) and the refined spin-pairing energy theory (RSPET). The hydration change of Ln3+(aq) affects the stability constants, because they are given by the reactions of Ln3+(aq) with CO32-(aq) and 2CO32-(aq), respectively. However, it does not affect their ratio of [β2(Ln(CO3)2-)/β1(LnCO3+)] which is the stepwise stability constant of Ln(CO3)2-(aq) for the reaction: LnCO3+(aq) + CO32-(aq) = Ln(CO3)2-(aq). Only when corrected for the hydration change of Ln3+(aq), the logβ1(Ln(CO3)+) values exhibit a regular convex tetrad effect across the series. Similarly corrected logβ2(Ln(CO3)2-) values also show a convex tetrad effect with a small break at Pr. The log [β2(Ln(CO3)2-)/β1(LnCO3+)] values are fairly constant, but display a small octad effect with convexity and the small break at Pr. The LnCO3+(aq) series appears to be an isomorphous complex series, but the Ln(CO3)2-(aq) series involves a structural change between the three lightest Ln members and the others. The RSPET analysis has been made for the tetrad effects after the correction for hydration change of Ln3+(aq), in which the small break at Pr in logβ2(Ln(CO3)2-) has also been corrected successfully. It was revealed that Racah E1 and E3 parameters decrease in the order that Ln3+(aq, octahydrate) >> LnCO3+(aq) > Ln(CO3)2-(aq). This corresponds to the nephelauxetic effect known in spectroscopic studies of Ln(III) complexes.