When sodium nitrite is added to the copper salt solutions, the solution turns green. This indicates the formation of a complex compound. Copper(II) nitrite alone could not be isolated, because it will quickly oxidize with air oxygen to nitrate(V).
In more basic solutions, poorly soluble copper basic nitrite Cu(NO2)2·xCu(OH)2 (where x may be from 1 to 3) is formed.
With a large excess of KNO2, K3Cu(NO2)5 was obtained with a complex structure.