1. Contribution from the Department of Chemistry, University of Oslo, Box 1033 Blindern, N-0315 Oslo, Norway, Anorganisch-chemisches Institut, Technische Universität München, Lichtenbergstrasse 4, D-85747 Garching, Germany, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK, and SINTEF, Applied Chemistry, Department of Hydrocarbon Process Chemistry, Box 124 Blindern, N-0314 Oslo, Norway