The ligands 4-amino-3-thioxo-1,2,4-triazin-5(4H)-one and its 2- and S-methyl derivatives have been synthesized and characterized. These ligands were reacted with Zn(II), Cu(II) and Ni(II)-chloride salts and their analytical dissociation constants were determined in dioxane/water (1/1,v/v) mixtures. Some of these coordination complexes were isolated at two different pH values. The coordination complexes synthesized at pH greater than or equal to 13.0 value show 5-one <----> 5-ol tautomerism. The metallated products have been characterized by using analytical data and IR spectroscopy.