Dimethoxy(trisyl)borane R3CB(OMe)2 (R = Me3Si throughout this abstr.) was hydrated to the borate (Et2O)LiH3BCR3 (I) by LiAlH4 in ether. R3CBH(OR1) (R1 = Me, Et, Me2CH, H) are formed in the reaction of I with R1OH by oxidative removal of two H-atoms. The removal of only one H atom from I can be achieved with either Me3NHCl or I2, yielding (R3CBH2)2 (II), which forms the adduct R3CBH2.PMe3 with PMe3. Starting from I, the borates (TMEDA)LiH3BCR3 and ClZr(H3BCR3)3.0.5Et2O (III) were prepd. by reaction with Me2NCH2CH2NMe2 and ZrCl4, resp.; the trisylborate groups are bonded to the Zr atom via H3-triple bridges. The crystal structures of II and III were detd.