1. Department of Inorganic Chemistry, Palacký University, CZ-77147 Olomouc, Czech Republic, Institute of Inorganic Chemistry, Slovak Technical University, SK-812 37 Bratislava, Slovakia, National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, and 1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany