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 CO₃^2-(aq) and 2CO₃^2-(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) + CO₃^2-(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.