1. Contribution from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K., Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, U.K., School of Chemistry, University of Hyderabad, Hyderabad 500 046, India, and DRAL, ISIS Facility, Chilton, Didcot, Oxon OX11 OQX, U.K.