ABSTRACT
Nitroso dyes used as biological stains contain a nitroso group in an ortho position to a phenolic hydroxyl. As such compounds exist as equilibrium mixtures of tautomers, as shown below for naphthol green Y, they can also be regarded as quinonoximes. The ortho -substitution pattern results in nitroso dyes being readily able to chelate a variety of transition metal ions, yielding colored metal coordination products. The structural characteristic of nitro dyes as narrowly defined is the presence of a nitro group in the ortho position to an electron donor. For most nitro dyes used as biological stains, this donor is a phenolic hydroxyl. The strongly electron accepting character of the nitro group results in ionization of these hydroxy groups at relatively low pH, hence their being used as acid (i.e. anionic) dyes.
