The reaction of UCl4 with sodium N,N-dimethylaminodiboranate, Na(H3BNMe2BH3), in diethyl ether affords the uranium(III) product U(H3BNMe2BH3)(3), which has been crystallized as two different structural isomers from pentane and toluene, respectively. The isomer crystallized from pentane is a polymer in which each uranium center is bonded to three chelating H3BNMe2BH3- (DMADB) ligands and to one hydrogen atom from a neighboring molecule so as to form an intermolecular B-H-U bridge; each uranium center is coordinated to 13 hydrogen atoms. The isomer crystallized from toluene is also polymeric, but the uranium atoms are coordinated by two chelating DMADB ligands and two bridging DMADB ligands bound in a U(kappa H-3-H3BNMe2BH3-kappa H-3)U fashion, so that each uranium atom is 14-coordinate. When the reaction of UCl4 with Na(H3BNMe2BH3) is conducted in tetrahydrofuran (thf) or 1,2-dimethoxyethane (dme), the adducts U(H3BNMe2BH3)(3)(thf) and U(H3BNMe2BH3)(3)(dme) are obtained. The rate of reduction from U-IV to U-III is correlated with the donor ability of the solvent, the relative rates being Et2O > thf > dme. The addition of trimethylphosphine to U(H3BNMe2BH3)(3)(thf) generates U(H3BNMe2BH3)(3)(PMe3)(2). This compound slowly decomposes at room temperature over several months to yield the new borane PMe3BH2NMe2BH3, mu-(N, N-dimethylamido)pentahydro(trimethylphosphine)diboron. Single crystal X-ray diffraction studies, H-1 and B-11 NMR spectra, IR data, and field ionization mass spectra for the uranium complexes are reported.