The activation of B-H and B-Cl bonds in boranes by base-stabilized low-valent silicon compounds is described. The reaction of the amidinato amidosilylene-borane adduct [L{Ar(Me3Si)N}SiBH3] [1; L = PhC(N tBu)2, and Ar = 2,6- iPr2C6H3] with MeOTf in toluene at room temperature formed [L{Ar(Me3Si)N}SiBH2OTf] (2). [LSiN(SiMe3)Ar] in compound 2 then underwent a B-H bond activation with BH2OTf in refluxing toluene to afford the B-H bond activation product [LB(H)Si(H)(OTf){N(SiMe3)Ar}] (3). On the other hand, when compound 2 was reacted with 4-dimethylaminopyridine in refluxing toluene, another B-H bond activation product [(μ-κ1:κ1-L)B(H)(DMAP)Si(H){N(Ar)SiMe3}]OTf (4) was afforded. Mechanistic studies show that "(μ-κ1:κ1-L)B(H)(OTf)Si(H){N(Ar)SiMe3}" (2A) is the key intermediate in the reactions mentioned above. The formation of 2A is further evidenced by the activation of the B-Cl bond in PhBCl2 by the amidinato silicon(I) dimer [LSi:]2 to form the B-Cl bond activation product [(μ-κ1:κ1-L)B(Cl)(Ph)Si(Cl)]2 (6). Compounds 2-4 and 6 were characterized by nuclear magnetic resonance spectroscopy and X-ray crystallography.