Trinuclear versus dinuclear heterodimetallic (UO2+)-O-V center dot center dot center dot Co2+ complexes were selectively assembled via a cation-cation interaction by tuning the ligand. The trimeric complex 2, with a linear [Co-OvUvO-Co] core, exhibits magnetic exchange and slow relaxation with a reversal barrier of 30.5 +/- 0.9 K providing the first example of a U-Co exchange-coupled SMM.