1. Institute of Inorganic Chemistry, Rheinische Friedrich-Wilhelms-Universität, Gerhard-Domagk-Straße 1, D-53121 Bonn, Germany
2. Mulliken Center for Theoretical Chemistry, Rheinische Friedrich-Wilhelms-Universität, Beringstr. 4, D-53121 Bonn, Germany
3. Chemistry Department, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland