Bromide, sulfate, selenite and selenate were found to form bridges in the cavity of mononuclear and binuclear Obisdien:Zn(II) and Obisdien:Mg(II) complexes in aqueous solution at 25.0 °C and m = 0.100 M (KCl). Potentiometric equilibrium measurements were used to determine the formation constants of the species formed in these systems in the pH range 2-12. For the Obisdien: Zn2+:anions complexes, we have established the existence of four species for bromide ion, five species for sulfate ion, six species for selenite ion and four species for selenate ion. We found that the hydroxo binuclear complexes predominate over the others species, and the binding strength decreases in the sequence: SeO4(2-) > SeO3(2-) > SO4(2-) > Br-. The Obisdien:Mg(II) system presents three species in case of binding with bromide ion, four with sulfate ion, five with selenite ion and five with selenate ion. Variation of the binding strength in the Obisdien:Mg(II) complexes is the same as in the Obisdien:Zn(II) complexes: SeO4(2-) > SeO3(2-) > SO4(2-) > Br-. Molecular mechanics calculations show that the dizinc(II)- and dimagnesium(II)-Obisdien complexes adopt several low energy conformations differing in their Zn to Zn and Mg to Mg separations that allow the coordination of SeO4(2-), SeO3(2-), SO4(2-), Br- and OH- anions.
selenite; selenate; sulfate; Obisdien complexes
