1. Contribution from the Institut für Anorganische Chemie, J.W. Goethe-Universität Frankfurt, Marie-Curie-Strasse 11, D-60439 Frankfurt (Main), Germany, and Kimika Fakultatea, Euskal Herriko Unibertsitatea, and Donostia International Physics Center DIPC, P. K. 1072, 20080 Donostia Euskadi, Spain