1. University of Göttingen, Institute of Inorganic Chemistry, Tammannstrasse 4, D-37077 Göttingen, Germany
2. Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Straße 17, A-1090 Vienna, Austria
3. Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland