1. Contribution from the Department of Chemistry, 225 Prospect Street, Yale University, New Haven, Connecticut 06520-8107, Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy, and LSDSMS (UMR 5636 CNRS), cc 14, Institut Gerhardt, Université Montpellier 2, F-34095 Montpellier Cedex 5, France