1. School of Crystallography, Birkbeck College, University of London, Malet Street, London, U.K. WC1E 7HX, Department of Chemistry, University College, University of London, 20 Gordon Street, London, U.K. WC1H 0AJ, and Davy Faraday Research Laboratory, Royal Institution, 21 Albemarle Street, London, U.K. W1X 4BS