Bis[tri(tert.-butyl)silanolato]tantalum(V) trichloride reacts with LiMe to give trimethyl-bis[tri- (tert.-butyl)silanolato]tantalum(V), 6. Its molecular structure shows a trigonal-bipyramidal coordination of the Ta atom with the Me groups in the equatorial plane. Its reaction with catecholborane produces preferably B-methyl-catecholborane besides some phenylene-1,2-dioxo-bis(1,3,2-dioxaborolane), 9, and H3 B・ THF. Similarly, the reaction of 6 with dimeric 9-borabicyclononane (9-BBN) generates preferable tri(tert.-butyl)siloxy-9-BBN (75%) and Me-9-BBN (25%). In contrast, cleavage of the Ta-O bonds is the preferred route in the treatment of 6 with H3B ・ THF. 11B NMR shows the formation of (tBu3SiO)2BH and (tBu3SiO)3B or (tBuSiO)3B3O3. No tantalum hydride could be isolated.