1. Contribution from the Beckman Institute, 139-74, California Institute of Technology, Pasadena, California 91125, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK, Department of Chemistry, Stanford University, Stanford, California 94305-5080, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, and Department of Chemistry, University of Arizona, Tucson, Arizona 85721