1. Department of Chemistry, University of SS. Cyril and Methodius, SK-917 01 Trnava, Slovakia, Department of Inorganic Chemistry, Slovak Technical University, SK-812 37 Bratislava, Slovakia, Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000, and Eduard-Zintl-Institute of Inorganic and Physical Chemistry, Materialscience, Darmstadt University of Technology, D-64287 Darmstadt, Germany