Solving the 170-Year-Old Mystery About Red-Violet and Blue Transient Intermediates in the Gmelin Reaction

The Gmelin reaction between nitroprusside and sulfides in aqueous solution is known to produce two transient intermediates with distinct colors: an initial red-violet intermediate that subsequently converts into a blue intermediate. In this work, we use a combination of multinuclear ((17) O, (15) N, (13) C) NMR, UV/Vis, IR spectroscopic techniques and quantum chemical computation to show unequivocally that the red-violet intermediate is [Fe(CN)5 N(O)S](4-) and the blue intermediate is [Fe(CN)5 N(O)SS)](4-) . While the formation of [Fe(CN)5 N(O)S](4-) has long been postulated in the literature, this study provides the most direct proof of its structure. In contrast, [Fe(CN)5 N(O)SS)](4-) represents the first example of any metal coordination complex containing a perthionitro ligand. The new reaction pathways found in this study not only provide clues for the mode of action of nitroprusside for its pharmacological activity, but also have broader implications to the biological role of H2 S, potential reactions between H2 S and nitric oxide donor compounds, and the possible biological function of polysulfides.