1. Contribution from the Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, British Columbia V8W 3V6, Canada, Institutionen för Oorganisk kemi, Chalmers Tekniska Högskola, SE-412 96, Göteborg, Sweden, Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, and Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X7, Canada