1. Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812, Japan, Dipartimento di Chimica, via Elce di Sotto 8, 06100 Perugia, Italy, and Institut de Chimie Minérale et Analytique, Université de Lausanne, BCH, CH-1015, Lausanne, Switzerland