Acknowledgements
Financial support of this work from the DFG and FCI is
gratefully acknowledged. R.K.O.S. wishesto thank the Swiss
National Science Foundation and the Swiss Federal Office
for Education and Science for a TMR-fellowship (No. 83
EU-046320) and J.M. thanks the Land Nordrhein-Westfalen
for a fellowship. This work was also part of a COST-D8
collaboration with the group of H. Sigel, Basel, Switzerland.