1. Department of Chemistry, University of Konstanz, Fach M728, D-78457 Konstanz, Germany, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznañ, Poland, and Institute of Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany