1. Centre de Recherches Paul Pascal (CNRS), Av. Schweitzer, F-33600 Pessac, France, Departement of Inorganic and Analytical Chemistry, L. Eötvös University, P.O. Box 32, H-1518 Budapest 112, Hungary, and Laboratoire des IMRCP, UMR 5623, Université Paul Sabatier, 31062 Toulouse Cedex 09, France