1. Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India, Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1B 3X7, and Anorganische und Analytische Chemie der Universität Gesamthochschule Paderborn, D-33098 Paderborn, Germany