1. Contribution from the Department of Chemistry, University of Manchester, Oxford Road, Manchester, U.K. M13 9PL, Chemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York 11973-5000, Departamento de Química Orgánica, ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain, and EPSRC X-ray Crystallography Service, Department of Chemistry, University of Southampton, Highfield, Southampton, U.K. SO17 1BJ