1. Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Bl.11, 1113 Sofia, Bulgaria
2. Department of Chemistry, University of Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland
3. Institut de Chimie Inorganique et Analytique, Universite de Fribourg Suisse, Perolles, CH-1700 Fribourg, Switzerland