Reaction of ctc-OsBr 2 (RaaiR ' ) 2 [RaaiR ' =1-alkyl-2-(arylazo)imidazole, p-R-C 6 H 4 -N N-C 3 H 2 -NN-1-R ' , where R=H (a), Me (b), Cl (c) and R ' =Me (2), Et (3) and CH 2 Ph (4)] with 2,2 ' -bipyridine (bpy) in presence of AgNO 3 in EtOH followed by the addition of NH 4 PF 6 afforded a mixed ligand complex [Os(bpy)(RaaiR ' ) 2 ](PF 6 ) 2 . The structure of the complex, in one case [Os(bpy)(MeaaiMe) 2 ](PF 6 ) 2 .4H 2 O, has been confirmed by X-ray crystallography. The complexes are diamagnetic (low spin d 6 , s=0) and they show intense MLCT transition in the visible region (480-525 nm) and a weak transition at longer wavelength (>850 nm) in CH 3 CN solution. Cyclic voltammetry of the complexes show two metal oxidation, Os(II)/Os(III) at 0.72-0.76 V and Os(III)/Os(IV) at 1.34-1.42 V and three successive ligand reductions.