Reaction Of UCl4 and excess Me(3)Sil in acetonitrile provides a convenient route to [Ul(4)(MeCN)(4)] (1), which was isolated in excellent yield and crystallized in acetonitrile as the ion pair complex [Ul(2)(MeCN)(7)][Ul(6)] (2). Compound 1 was transformed in pyridine (py) and dimethylformamide (dmf) into the Lewis base adducts [Ul(4)(py)(3)] (3) and [U-4(dMf)(6)] (4). Crystals of 3 and [U(dmf)(9)]l(4) (5) were obtained by slow diffusion of diethyl ether into pyridine or dmf solutions of 1; compound 5 is the first tetracationic {U4+} entity to have been crystallographically characterized.