1. Institute of Organic Chemistry, University of Leipzig, Johannisallee 29, D-04103 Leipzig, Germany, A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str.28, GSP-1, V-334, Rus-117813 Moscow, Russia, and CNR, Department of Chemistry of the Politecnico, via Mancinelli 7, I-20131 Milan, Italy