1. Department of Chemistry, University College of Science, University of Calcutta, 92 A.P.C. Road, Kolkata 700 009, India
2. R.S.I.C, Bose Institute, Kolkata 700 009, India
3. School of Chemistry, The University of Reading, P. O. Box 224, Whiteknights, Reading, RG6 6AD, UK
4. Departament de Química Inorgànica, Universitat de Barcelona, 08028 Barcelona, Spain