A variety of crown ether complexes of uranium have been studied, but little evidence existed until recently to support the idea that direct coordination of uranium to the oxygens of the crown cavity had been achieved. Although a lanthanum nitrate crown ether complex had been found to possess the lanthanum coordinated in this manner,3 only recently has a uranium(IV) complex been shown to exhibit this type of coordination.4 The factors influencing the binding of uranium to the crown cavity still remain somewhat uncertain, however. Similarly, the extent that such coordination is oxidation state specific remains unknown. For these reasons, as well as the close similarity of La(III) and U(III) ionic radii and the limited information on chemical properties of U(III) compounds, we have undertaken the study of uranium(III)-crown. © 1979, American Chemical Society. All rights reserved.