1. Contribution from the Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, 3000 Bern 9, Switzerland, Indiana University, Department of Chemistry, 800 East Kirkwood Avenue, Bloomington, Indiana 47405-7102, Institute Laue Langevin, Avenue des Martyrs, B. P. 156, F-38042 Grenoble Cedex 9, France, and Hahn-Meitner Institut, Glienicker Strasse 100, D-14109 Berlin-Wannsee, Deutschland