1. C.N.R.S., U.P.R. 01332 and 209, F‐92195 Meudon, France
2. L.U.R.E., Centre Universitaire Paris‐Sud, F‐91405 Orsay, France
3. Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, SF‐02150 Espoo, Finland
4. Department of Chemistry, Helsinki University, SF‐00014 Helsinki, Finland