Bi-functionalizion of Wells-Dawson-type polyoxotungstates with exo-coordinated rare earth metal ions (M = DyIII, YIII) and phenylphosphonate induces the formation of a separate second lacunary site in the pre-functionalized [α2-P2W17O61(PhPO)2]6- precursor. The resulting [{(H2O)xM2(α2,α2'-P2W16O60)(PhPO)2}2]8- dimer clusters are stable in solution, exhibit interesting electrochemical behavior, and, for the DyIII derivative, single ion-magnet characteristics.