The complexation equilibria between Mg2+ and D-gluconate (Glue) ions are of particular importance in modeling the chemical speciation in low- and intermediate-level radioactive waste repositories. NMR measurements and potentiometric titrations conducted at 25 degrees C and 4 M ionic strength revealed the formation of the MgGluc(+), MgGlucOH(0), MgGluc(OH)(2)(-), and Mg(3)Gluc(2)(OH)(4)(0) complexes. The trinuclear species provides indirect evidence for the existence of multinuclear magnesium(II) hydroxido complexes, whose formation was proposed earlier but has not been confirmed yet. Additionally, speciation calculations demonstrated that MgCl2 can markedly decrease the solubility of thorium(IV) at low ligand concentrations. Regarding the structure of MgGluc(+), both IR spectra and density functional theory (DFT) calculations indicate the monodentate coordination of Gluc(-). By the potentiometric data, the acidity of the water molecules is higher in the MgGluc(+) and MgGlucOH(0) species than in the Mg(H2O)(6)(2+) aqua ion. On the basis of DFT calculations, this ligand-promoted hydrolysis is caused by strong hydrogen bonds forming between Gluc(-) and Mg(H2O)(6)(2+). Conversely, metal-ion-induced ligand deprotonation takes place in the case of calcium(II) complexes, giving rise to salient variations on the NMR spectra in a strongly alkaline medium.