Reactions of OsCl_2(PPh_3)_3(1) with bidentate nitrogen-based ligands at room temperature led to the formation of complexes OsCl_2(PPh_3)_2(diamine)(2~6) and OsCl_2(PPh_3)_2(Phen*)(7~11). Complexes 2~11 showed high activity in the catalytic dehydrogenation of ammonia borane at 60 ℃, in THF/DME(V∶V=1∶1.6) solution with the catalyst loading of 5mol%. Among all the catalysts, compound 4 demonstrated the highest catalytic activity, which represents the most efficient osmium catalyst in catalytic dehydrogenation...