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