The magnetic and optical properties of the divalent state of europium make this ion extremely attractive for use in materials,[1] catalysis,[2] luminescence,[3] magnetic,[4] and diagnostic-medical [5] applications. A major hindrance to the use of EuII in many of these applications is the extreme propensity of the ion to oxidize to EuIII, especially in aqueous solution. Research efforts aimed at increasing the stability of aqueous EuII have yielded ...