1. Contribution from the Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands, Laboratory for Inorganic Chemistry, Faculty of Applied Sciences, Delft University of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands, and N. S. R. Center for Molecular Design, Synthesis and Structure, Department of Physical Chemistry/Solid-state NMR, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands.