1. Inst. fur Anorg. Chemie, Johann Wolfgang Goethe University, Marie-Curie-Str. 11, D - 60439 Frankfurt/Main, Mertonviertel, Oberursel, Dow Corning Corporation, Midland, Michigan 48686-0995, and Dow Corning Asia, Ltd., 603 Kishi, Yamakita, Kanagawa 258-0112, Japan