The reaction between the osmahexaborane [2,2,2-(PPh3)(2)(CO)-nido-2-OsB5H9] (1) and bases such as PPh3, PPh2Me, and PMe3 in refluxing CH2Cl2 affords unique adducts of the type [2,2,2-(PPh3)(2)(CO)-nido-2-OsB4H7-3-BH2. PR3] (2) for which spectroscopic data suggest the presence of a pendent boryl group. This was confirmed by a crystal structure determination for the PPh2Me adduct which shows that 2 is a nirlo-osmapentaborane with a terminal BH2. PPh2Me moiety on a basal boron atom adjacent to the metal. The reaction is reversible in the case of PPh3 and to a lesser extent PPh2Me, but not for PMe3. Heating the PPh3 adduct affords the osmahexaborane 1, liberating PPh3, but degradation to the osmapentaborane [2,2,2-(PPh3)(2)(CO)-nido-2-OsB4H8] (6) and BH3. PPh3 competes. The tendency to degrade to phosphine borane increases markedly down the series R-3 = Ph-3, Ph2Me, and Me-3. When the bidentate bases [1,2-(PPh2)(2)(CH2)(2)] and [1,3-(PPh2)(2)(CH2)(3)] (abbreviated as dppe and dppp, respectively) are used, two major products are observed in each case. One (3a) [2,2,2-(PPh3)(2)(CO)-nido-2-OsB4H7-3-(BH2. dppe)] (or 3b, BH2. dppp) is analogous to 2 with one end of the bidentate phosphine donor uncoordinated. In the other (4a) [2,2-(PPh3)(CO)-nido-2-OsB4H7-eta(2)-3,2-(BH . dppe)] (or 4b, BH2. dppp), the free end of the bidentate ligand has replaced a PPh3 group on Os. In the reaction of [(PPh2)(2)(CH2)] (abbreviated as dppm) with 1, only a species analogous to 3 is observed. The species 3b, the one involving dppp, has been further modified at the free phosphine end of the ligand, to form [2,2,2-(PPh3)(CO)-nido-2-OsB4H7-3-(BH2. dppp . BH3)] (5).