To a near boiling 35 mL of 10 % ammonia add 2.5 g of chloropentaamminecobalt(III) chloride with stirring. Then immediately filter the mixture and cool the solution to 0°C and add as much of 10 % HCI until neutralized and then 2.5 g of sodium nitrite. Add 2.5 mL of 20 % HCI with stirring. Allow the flask to be on the ice for 2 h with stirring and then filter with washing with cold ethanol. Dry and store in a container, the compound with isomerize into [Co(NH3)5NO2]Cl2.
[Co(NH3)5CI]Cl2 + NaNO2 = [Co(NH3)5NO2]Cl2 + NaCl
Filtration and washing
The dry product