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, U.K., Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, and Department of Chemistry, University of Arizona, Tucson, Arizona 85721