Stoichiometries and stability constants for the cerium(IV) carbonate complexes in dilute aqueous solution

Complexation between cerium(IV) and carbonate ions in aqueous solution has been studied by UV/Visible absorption spectroscopy, O-17 NMR spectroscopy and potentiometric titration, and it is shown that in dilute solutions at pH 8.8 and 10, both 1 : 1 and 1 : 2 (metal : ligand) complexes are formed. This contrasts with the behaviour of the corresponding Th(IV) complex, where the dominant species is the pentacarbonato complex. From the NMR spectra, it is suggested that these species involve bidentate binding of carbonate ion by the metal, while potentiometric data provided accurate formation constants and indicated the dominant species to be the 1 : 2 complex.