1. Contribution from the Department of Inorganic, Analytical and Applied Chemistry, the Laboratory of X-ray Crystallography, and the Department of Biochemistry, University of Geneva, CH-1211 Geneva 4, Switzerland, the Institute of Analytical and Inorganic Chemistry, University of Lausanne, CH-1015 Lausanne, Switzerland, and Pharma Division, F. Hoffmann-La Roche Ltd., CH-4002 Basel, Switzerland