The Mossbauer, magnetic susceptibility and EXAFS data for the oxo-bridged heterometallic acetylacetonate Fe3YO6R3[Y-11(C5O2H7)(33)] (R = C5H7) are analyzed in terms of a bound-to-free model describing the transition from the state of "delocalized valence", denoted Fe-deloc((3-delta)+), in the magnetically disordered "paramagnetic" complex at 300 K to the states of "localized valence", denoted Fe-loc(2+) and Fe-loc(3+), in the magnetically ordered complex at 4.2 and 77 K. (C) 2003 Kluwer Academic Publishers.