1. Department of Inorganic Chemistry, University of La Laguna, 38204 La Laguna, Tenerife, Canary Islands, Spain; Laboratory of Equilibria in Solution, School of Chemistry, Faculty of Sciences, Central University of Venezuela, Caracas, Venezuela; and Department of Inorganic Chemistry, University of the País Vasco, Apartado 644, E-48080 Bilbao, Spain