Reaction of nitricoxide with N,N′-ethylenebis(acetylacetoneiminato)iron(II), [Fe(acacen)], led to a mixed-valence tetranuclear iron complex, [ON–Feacacen(NO)2}]3Fe}, containing, as seen from the X-ray analysis, three ironnitrosyl complexes binding the fourth iron in an octahedral cavity with six oxygen donor atoms.