1. A.N. Nesmeyanov Institute of Organo-Element Compounds, Russian Academy of Sciences, 117813, Moscow, Vavilov 28, Russian Federation, Avecia, Ltd., P.O. Box 521, Leeds Rd. Huddersfield, West Yorkshire, HD2 1GA, UK, and Department of Chemistry, King's College London, Strand, London, WC2 R2LS, UK