The complex OsH2Cl2((PPr3)-Pr-i)(2) reacts with pinacolborane, Me2NH-BH3, and (BuNH2)-Bu-t-BH3 to give the complexes OsH2Cl{eta(2)-HBOC(CH3)(2)C(CH3)(2)OBpin}((PPr3)-Pr-i)(2) and OsH2Cl(eta(2)-(HBNRR2)-R-1)((PPr3)-Pr-i)(2) (R-1 = R-2 = Me; R-1 = H, R-2 = Bu-t) containing monosubstituted alkoxy- and amidoborinium cations coordinated as sigma-B-H ligands. The process is proposed to take place via the electrophilic 14-valence-electron fragment OsHCl((PPr3)-Pr-i)(2), which promotes hydride transfer from the corresponding borane to the osmium atom.