1. Contribution from the Structural Chemistry Research Group of the Hungarian Academy of Sciences, Eötvös University, Pf. 117, H-1431 Budapest, Hungary, Institute of General and Analytical Chemistry, Budapest Technical University, H-1521 Budapest, Hungary, and IRSAMC, Laboratoire de Physique Quantique CNRS UMR 5626, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France