Abstract The dimethylaminopyridine (DMAP) promoted reaction between [Os(bpy) 2(CO)(OTf)]OTf (where OTf = CF 3 SO 3 - ) and methylene chloride is reported. C–Cl bond breaking of a solvent molecule leads to the formation of the [Os(bpy) 2(CO)(Cl)]OTf complex. The reactivity and redox properties of [Os(bpy) 2(CO)(OTf)]OTf were investigated by means of room- and low-temperature electrochemical experiments. In CH 2Cl 2, at low temperature, the complex undergoes two 1e electrochemical and chemical reversible reductions ( E r E r mechanism), but at room temperature a more complex electrochemical mechanism is observed, leading to the electro-synthesis of [Os(bpy) 2(CO)(Cl)]OTf via electrochemical reversible and chemical irreversible reduction processes ( E r C i mechanism). The DMAP nucleophilicity was used to produce the new [Os(bpy) 2(CO)(Br)]OTf and [Os(bpy) 2(CO)(I)]OTf complexes which have been fully characterized.