1. Contribution from the School of Chemistry, Physics and Environmental Sciences, University of Sussex, Brighton, Sussex BN19QJ, U.K., Fachbereich Chemie, Universität Kaiserslautern, Erwin-Schrödinger-Strasse Gebaeude 54, D-67663 Kaiserslautern, Germany, and Department of Inorganic Chemistry, Technical University of Budapest, Gellért tér 4, H-1521 Budapest, Hungary