The reaction of a slurry of BaBr2 in a minimal amount of tetrahydrofuran (THF) with 2 equiv of Na(H3BNMe2BH3) in diethyl ether followed by crystallization from diethyl ether at -20 degrees C yields crystals of Ba(H3BN-Me2BH3)(2)(Et2O)(2) (1). Drying 1 at room temperature under vacuum gives the partially desolvated analogue Ba(H3BNMe2BH3)(2)(Et2O)(x) (1') as a free-flowing white solid, where the value of x varies from <0.1 to about 0.4 depending on whether desolvation is carried out with or without heating. The reaction of 1 or 1' with Lewis bases that bind more strongly to barium than diethyl ether results in the formation of new complexes Ba(H3BNMe2BH3)(2)(L), where L = 1,2-dimethoxyethane (2), N,N,N',N'-tetramethylethylenediamine (3), 12-crown-4 (4), 18-crown-6 (5), N,N,N',N'-tetraethylethylenediamine (6), and N,N,N',N '',N ''-pentamethylethylenetriamine (7). Recrystallization of 4 and 5 from THF affords the related compounds Ba(H3BNMe2BH3)(2)(12-crown-4)(THF)center dot THF (4') and Ba-(H3BNMe2BH3)(2)(18-crown-6)center dot 2THF (5'). In addition, the reaction of BaBr2 with 2 equiv of Na(H3BNMe2BH3) in the presence of diglyme yields Ba(H3BNMe2BH3)(2)(diglyme)(2) (8), and the reaction of 1 with 15-crown-5 affords the diadduct [Ba(15-crown-5)(2)][H3BNMe2BH3](2) (9). Finally, the reaction of BaBr2 with Na(H3BNMe2BH3) in THF, followed by the addition of 12-crown-4, affords the unusual salt [Na(12-crown-4)(2)][Ba(H3BNMe2BH3)(3)(THF)(2)] (10). All of these complexes have been characterized by IR and H-1 and B-11 NMR spectroscopy, and the structures of compounds 1-3, 4', 5', and 6-10 have been determined by single-crystal X-ray diffraction. As the steric demand of the Lewis bases increases, the structure changes from polymers to dimers to monomers and then to charge-separated species. Despite the fact that several of the barium complexes are monomeric in the solid state, none is appreciably volatile up to 200 degrees C at 10(-2) Torr.