4,5-Dihydro-5-oxo-[ 1,2,4]triazolo-[1,5-a]pyrimidine (5HtpO) and
4,7-dihydro-7-oxo-[1,2,4]triazolo-[1,5-a]pyrimidine(7HtpO) have been synthesized by the condensation of 3-amino-[1,2,4]triazole with a reagent bearing the three carbon atoms that close the six-membered ring in a strongly acidic (7HtpO) or strongly basic (5HtpO) medium. The crystal structures of these compounds have been determined by X-ray diffraction, exhibiting the N4-H tautomer, as expected from the RHF/AM1 calculations. The different possibilities for binding metal ions are discussed from the MO results.