Tuning of Polyoxopalladate Macroanionic Hydration Shell via Countercation Interaction

Three types of macroanion-countercation interactions in dilute solution, decided by the strength of electrostatic attraction and the change of hydration shells are reported: minor interaction between macroanions [MO8Pd12(SeO3)8]6- (M = Zn2+ or Ni2+) and monovalent cations (Na+, K+, Rb+, Cs+), leaving their hydration shells intact (solvent-separated ion-pairs); strong binding between macroanions and divalent cations (Sr2+, Ba2+) to form solvent-shared ion-pairs with partial dehydration; very strong electrostatic attraction between macroanions and Y3+ ion with contact ion-pairs formation by severely breaking their original hydration shells and forming new ones. Additionally, divalent cations can help the macroanions self-assemble into hollow spherical blackberry structures through counterion-mediated attraction, whereas macroanions with mono- or trivalent cations only stay as discrete ions due to either weak interaction or a small number of bound countercations.