SOME TETRAHYDROBORATE DERIVATIVES OF ALUMINUM - CRYSTAL-STRUCTURES OFDIMETHYLALUMINUM TETRAHYDROBORATE AND THE ALPHA-PHASE AND BETA-PHASE OF ALUMINUM TRIS(TETRAHYDROBORATE) AT LOW-TEMPERATURE

The structures of several aluminium tetrahydroborates have been invest igated, primarily by X-ray diffraction of single crystals at low tempe ratures. The structure of dimethylaluminium tetrahydroborate has been shown to consist of helical polymeric chains in which Me(2)Al and BH4 units alternate. The BH4 groups exhibit bidentate ligation with respec t to each of the adjacent metal atoms in a manner reminiscent of [MeZn BH(4)] and Be(BH4)(2). Solid aluminium tris(tetrahydroborate) exists i n two phases with a transition temperature in the range 180-195 K. Eac h phase is made up of discrete Al(BH4)(3) units in which the angle bet ween AlB2 and Al(mu-H)(2)B planes is close to 90 degrees and which con form within the limits of experimental error to D-3h symmetry; in the alpha phase these molecular units are disposed around a 2, crystallogr aphic screw axis.