1. Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France, Department of Structural Chemistry (DCSSI), University of Milan, 21, Via G. Venezian, 20133 Milan, Italy, and Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France