1. Department of Inorganic, Analytical and Applied Chemistry and Department of Physical Chemistry, University of Geneva, 30 quai E. Ansermet, CH-1211 Geneva 4, Switzerland, Laboratory of X-ray Crystallography, University of Geneva, 24 quai E. Ansermet, CH-1211 Geneva 4, Switzerland, and Institute of Molecular and Biological Chemistry, Swiss Federal Institute of Technology, BCH 1402, CH-1015 Lausanne, Switzerland