FT-IR spectroscopy of adsorbed NO was used to investigate the electronic and geometric structure of the extraframework iron species formed upon thermal activation of four Fe-silicalite samples with Si/Fe ratios ranging from 25 to 150. An heterogeneous family of grafted Fe2+ species, classified in terms of their first coordination shell, i.e. as L2N"Fe2+ (where L are the framework SiO- groups chemically linked to the Fe species and N are the framework oxygen atoms of the vicinal Si-O-Si bridges interacting with Fe centres via electrostatic forces) has been detected as the primary product of the Fe migration. Depending upon the number of N ligands (3, 2 or 1) the Fe2+ species can adsorb one, two, and three NO molecules, respectively, with formation of well-defined nitrosylic complexes. L2N"Fe2+ (n = 1 and 2) are supposed to be isolated species, while L2N3Fe2+ are probably dimeric or polymeric clusters. Fe3+ extraframework species can be reduced to Fe2+ by prolonged outgassing in vacuo. The formation of large Fe2O3 clusters is inferred for the most concentrated sample. 0, dissociates with formation of adsorbed oxygen on all extraframework Fe species at 573 K. At room temperature only dimeric or polymeric L2N3Fe2+ species are able to dissociate O-2. (C) 2002 Elsevier Science B.V. All rights reserved.