1. Department of Chemistry, Vanderbilt University, Nashville, TN 37235, U.S.A., Fax: (internat.) + 1‐615/343‐1234
2. X‐ray Crystallographic Laboratory, Chemistry Department, University of Minnesota, Minneapolis, MN 55455, U.S.A., Fax: (internat.) + 1‐612/626‐7541