Rinmann's green is:

• A. Cobalt thiocyanate
• B. Zinc cobalt nitrite
• C. Aluminum cobalt nitrite
• D. Ferric thiocyanate

Answer: (B)

Colors of metal complexes come from how ligands change the energy levels of the metal ion, producing specific light absorption. Rinmann's green is the green complex formed when cobalt is coordinated by nitrite in a zinc-containing medium. The nitrite ligands alter cobalt(II)’s electronic transitions just enough to reflect green light under the test conditions. The other possibilities yield different colors under the same conditions—ferric thiocyanate gives a deep red, cobalt thiocyanate appears blue, and the aluminum cobalt nitrite combination doesn’t produce the characteristic green. So the green color points to the zinc cobalt nitrite complex.