1. Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow, G1 1XL, U.K., Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, U.K., Supercomputer Computations Research Institute B-186, Florida State University, Tallahassee, Florida 32306-4052, and Institut für Anorganische Chemie, Tammannstrasse 4, D-37077 Göttingen, Germany