1. Department of Chemistry, King's College London, Strand, London WC2R 2LS, U.K., Johnson Matthey, P.O. Box 1, Belasis Avenue, Billingham, Cleveland TS23 1LB, U.K., EPSRC National Crystallography Service, University of Southampton, Highfield, Southampton SO17 1BJ, U.K., and Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, U.K.