1. Contribution from the Laboratorium für Anorganische Chemie, Eidgenössische Technische Hochschule, ETH Hönggerberg, CH-8093 Zürich, Switzerland, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba, Sendai 980-8577, Japan, and Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan