1. Institute of Physical and Theoretical Chemistry, J. W. Goethe University of Frankfurt, and Center for Biological Magnetic Resonance, Max-von-Laue-Straβe 7, D-60438 Frankfurt, Germany, Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, SK-84536 Bratislava, Slovakia, and Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany