1. Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland, Laboratory for Neutron Scattering, ETH Zürich and Paul-Scherrer Institute, CH-5232 Villigen PSI, Switzerland, Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany, Institut Laue-Langevin, BP156, 38042 Grenoble Cedex 9, France, and High Magnetic Field Laboratory, CNRS/MPI, B.P. 166, 38042 Grenoble, France