FTIR and diffuse reflectance UV-Vis spectroscopies were used to investigate the nature of the extra-framework iron species in Fe-Silicalite using NO as probe molecule. After removal of the template at 773 K, ferrous species present in two different coordinative states, namely Fe-A(2+) and Fe-B(2+) were mainly detected. The Fe-A(2+) species are highly coordinatively unsaturated and can form a [Fe-A(NO)(n)](2+) (with n > 2) polynitrosyl complex characterized by IR bands at 1914, 1880-1850, and 1808 cm(-1) and by an electronic transition at 23,000 cm(-1). Upon reducing the NO pressure, this complex is transformed into a [Fe-A(NO)(2)](2+) dinitrosyl characterized by a doublet of IR bands at 1839 and 1765 cm(-1). Fe-B(2+) is less coordinatively unsaturated and forms only [Fe-B(NO)](2+) mononitrosyl (absorbing in the IR at 1839 cm(-1) and at 26,500 and 17,500 cm(-1) in the UV-Vis). Upon dosage of H2O, the Fe-B(2+) species becomes able to uptake further NO ligands to give new polynitrosyl species spectroscopically indistinguishable from those formed on the Fe-A(2+) species. Based on these results, some hypotheses about the structure of Fe-A(2+) and Fe-B(2+) are advanced.