1. Contribution from the INSTM and Department of Chemistry, University of Modena and Reggio Emilia, via G. Campi 183, 41100 Modena, Italy, Laboratoire des Champs Magnetiques Intenses-CNRS, BP166, 25 Avenue des Martyrs, 38042 Grenoble Cedex 9, France, INSTM and Department of Chemistry, University of Florence, via della Lastruccia 3, 50019 Florence, Italy, and Laboratoire Louis Néel-CNRS, BP166, 25 Avenue des Martyrs, 38042 Grenoble Cedex 9, France