1. Contribution from the Department of Inorganic Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland, Laboratory of Crystallography, University of Geneva, Switzerland, Department of Inorganic Chemistry, University of Karlsruhe, Germany, Department of Physical Chemistry, University of Geneva, Geneva, Switzerland, and Laboratoire de Chimie-Physique Théorique and Institut de Recherches sur la Catalyse, CNRS, UPR 5401, France