From pH 4.1 to 8.9, in buffer solutions, rapid substitution in the ion pair [Os(NH3)(6)](3+).[Fe(CN)(6)](3-) leads to the formation of [Os(NH3)(5)Fe(CN)(6)], without net relocation of the electrons or structural rearrangement besides the replacement of NH3. Subsequent reactions are the formation of [Os-IV(NH3)(4)NH2(Fe-II(CN)(6))](-) and, on a much slower time scale, disproportionation to [Os-VI(NH3)(4)N(Fe-II(CN)(6))(-) and [Os-III(NH3)(5)(Fe-II(CN)(6))](-).