1. Contribution from the Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, PO Box 80083, 3508 TB Utrecht, The Netherlands, BP Amoco Chemicals Research Center, Mail Station E-1F, 150 West Warrenville Road, Naperville, Illinois 60563-8460, and the Chemistry Department, George Washington University, Washington, D.C. 20052