1. Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India, Departament de Química Inorgànica, Universitat de Barcelona, Diagonal, 647, 08028 Barcelona, Spain, Dipartimento di Scienze Chimiche, University of Trieste, 34127, Trieste, Italy, and Department of Physics, Jadavpur University, Jadavpur, Kolkata 700 032, India