1. Central Research and Development, E. I. DuPont de Nemours & Co., Inc., Experimental Station, Wilmington, Delaware 19880‐0328, USA, Fax: +1‐302‐695‐8281
2. Departament de Química Inorgànica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain, Fax: +34‐93‐490‐7725