Cupferron

Cupferron, the ammonium salt of N-nitroso-N-phenylhydroxylamine, is a common reagent for the complexation of metal ions. Its formula is NH4[C6H5N(O)NO]. The anion binds to metal cations through the two oxygen atoms, forming five-membered chelate rings.

Cupferron is prepared from phenylhydroxylamine and an NO+ source:
 * C6H5NHOH +  C4H9ONO  +  NH3   →  NH4[C6H5N(O)NO]  +  C4H9OH