1. Contribution from the Department of Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden, Inorganic Chemistry, The Ångström Laboratory, Uppsala University, P.O. Box 538, S-751 21 Uppsala, Sweden, Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015, S-750 07 Uppsala, Sweden, and Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden