1. Contribution from the Anorganisch Chemisch Laboratorium, Institute of Molecular Chemistry, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, Department of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K., Department of Chemistry, University of Massachusetts at Boston, Boston, Massachusetts 02125, and Department of Chemistry, Queen Mary and Westfield College, University of London, Mile End Road, London, E1 4NS, U.K.