Abstract Although mononuclear complexes were mainly obtained from Cu 2+ ion and phenyl- , ethyl-or benzyl-substituted thiazolo-1, 2, 4-triazine derivatives, unique metastable orange– yellow crystals [CuCl 2 (3a)] n (5a; Cu/3a= 1: 1) with the phenyl-substituted derivative 3a were obtained as a 1D coordination polymer through bis (μ-chloro) linkages in a 2-propanol solution. The crystals show dynamic and irreversible structural transformation into blue ...