Novel compositions of matter are disclosed having the formula UO.sub.2 AA'L.sub.n, wherein n is 0 or 1, and in which A and A' are anions whose conjugate acids have boiling points less than about 200.degree. C. and pK.sub.a values of 4.8 or less, L is a neutral ligand having a boiling point less than about 190.degree. C. and having an equilibrium constant for the exchange reaction with tetrahydrofuran of between about 10.sup.-3 and 10.sup.3, and the combination of A, A' and L satisfy five or six coordination sites of the central uranium atom.