Treatment of Si2Cl6 with [Et4N] [BCl4] in CH2Cl2 furnished the known products of a chloride-induced disproportionation reaction of the disilane, such as SiCl4, [Si(SiCl3)(3)](-), and [Si6Cl12.2Cl](2-). No Si B-bonded products were detectable. In contrast, the addition of Si2Cl6 to [Et4N][BI3Cl] afforded the Si-B adduct [Et4N] [I3SiBI3]. Thus, a quantitative Cl/I exchange at the silicon atom accompanies the trihalogenosilanide formation. [Et4N][I3SiBI3] was also accessible from a mixture of Si2I6, [Et4N]I, and BI3. According to X-ray crystallography, the anion [I3SiBI3](-) adopts a staggered conformation with an Si-B bond length of 1.977(6) A. Quantum-chemical calculations revealed a polar covalent Si-B bond with significant contributions from intramolecular dispersion interactions.