1. Contribution from the Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Institute of Physical and Theoretical Chemistry, J. W. Goethe University of Frankfurt, and Center for Biological Magnetic Resonance, 60439 Frankfurt, Germany, Department of Chemistry, Florida International University, Miami, Florida 33199, and Institute of Inorganic Chemistry, J. Maximilians University of Würzburg, 97074 Würzburg, Germany