1. Institut für Anorganische Chemie, Georg‐August‐Universität Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany, Fax: +49‐551‐393063
2. New address: Department of Inorganic Chemistry, Faculty of Pharmacy, Wroclaw Medical University, Szewska 38, 50‐139 Wroclaw, Poland