1. Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700 009, India, School of Chemistry, The University of Reading, P. O. Box 224, Whiteknights, Reading RG 66AD, United Kingdom, and Departament de Química Inorgànica, Universitat de Barcelona, Marti i Franques 1-11,08028-Barcelona, Spain