1. Département de Chimie, Université de Fribourg Suisse, Ch. du Musée 9, CH-1700 Fribourg, Switzerland, Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad.G.Bontchev Str. Bl.11, 1113 Sofia, Bulgaria, Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland, and Département de Chimie Physique, Université de Genève, Sciences II, 30, Quai Ernest-Ansermet, CH-1211 Genève 4, Switzerland