Structural characterization of a mononuclear uranyl complex with a tetradentate, thiophene-linked bis(3-hydroxy-N-methylpyridin-2-one) ligand reveals the most planar coordination geometry yet observed with this ligand class. The introduction of ethylsulfanyl groups onto the thiophene linker disrupts this planar, conjugated ligand arrangement, resulting in the formation of dimeric (UO2)(2)L-2 species in which each ligand spans two uranyl centers. Relative energy calculations reveal that this tendency toward dimer formation is the result of steric interference between ethylsulfanyl substitutents and linking amides.