1. Contribution from the Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K., Dipartimento di Chimica “G. Ciamician”, Universita di Bologna, via Selmi 2, I-40126 Bologna, Italy, and Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD, U.K.