Nitroprusside Color Mystery Resolved

A 170-year-old chemical color mystery has been solved with ¹⁷O NMR spectroscopy and other analytical techniques. The Gmelin reaction, first observed in the 1840s by German chemist Leopold Gmelin, is considered one of the most intense color-forming reactions. When nitroprusside ([Fe(CN)₅NO]^2–) and sulfide combine they produce a brilliant red-violet intermediate that converts rapidly to a deep-blue intermediate. The red-violet intermediate has been proposed to be [Fe(CN)₅(HSNO)]^3– or [Fe(CN)₅(SNO)]^4–, but its structure was never confirmed. The identity of the blue intermediate has remained unknown, as has that of its decomposition products. Gang Wu and coworkers at Queen’s University, in Ontario, have now used ¹⁷O, ¹⁵N, and ¹³C NMR along with UV-visible and infrared spectroscopy and quantum chemical computations to show that the red-violet intermediate is [Fe(CN)₅N(O)S]^4– and the blue molecule is [Fe(CN)₅N(O)SS)]^4– (Chem. Eur. J. 2015, DOI: 10.1002/chem.201503353). Nitroprusside is used to treat hypertension, and the new findings could aid understanding of its mode of action, Wu says.