Reaction of cyanide with hydroxamic acid iron complexes to distinguish trihydroxamates from simple monohydroxamates.

At high pH primary hydroxamic acid-iron complexes react rapidly with potassium cyanide to yield a deep blue iron complex. Secondary, or N-substituted, monohydroxamic acid-iron complexes also react with loss of the red color typical of these complexes but with no formation of the blue product. Under the same conditions, trihydroxamates do not react at a significant rate. The blue complex is similar in many respects to the previously described Fe(CN)5NO3- but is noteworthy in its stability to oxidation and extremely high pH. Lack of formation of the blue complex with secondary hydroxamates is attributed to the absence of inorganic hydroxylamine so that the nitrosyl group cannot be formed. Lack of reactivity of siderophore trihydroxamates is due to the much greater stability of their iron complexes. The reaction is a simple, convenient method of distinguishing primary, secondary, and siderophore trihydroxamic acids.