1. School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, U.K., Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K., X-ray Crystallographic Unit, Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K., and Institut Laue-Langevin, BP156, 38042 Grenoble Cedex 9, France