1. Contribution from the Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, CZ- 250 68 Řež near Prague, Czech Republic, Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany, and the Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, United Kingdom