Although mononuclear complexes were mainly obtained from Cu2+ ion and phenyl-, ethyl- or benzyl-substituted thiazolo-1,2,4-triazine derivatives, unique metastable orange-yellow crystals [CuCl2(3a)](n) (5a; Cu/3a = 1:1) with the phenyl-substituted derivative 3a were obtained as a 1D coordination polymer through bis(mu-chloro) linkages in a 2-propanol solution. The crystals show dynamic and irreversible structural transformation into blue crystals of [CuCl2(3a)(2)] (6a; Cu/3a = 1:2) not only in solution but also in vapor conditions. As 5a and 6a are ferromagnetic and paramagnetic, respectively, the transformation of 5a was monitored by magnetic susceptibility measurements in the solid state, and chi T of 5a decreased according to the color change. The XRD pattern of 5a also changed to a new aggregation state, ascribed to the disproportionation of the 1D network to form a mononuclear complex and a solvated metal ion in the crystalline state. This unusual 1D network is stabilized by intermolecular pi-pi stacking between the phenyl and the triazine moieties, and comparative studies of the ethyl and benzyl-substituted complexes show no polymeric structures. The coordination site of all the thiazolo-1,2,4-triazine derivatives in this work was found to be the imino nitrogen as a monodentate coordination site by X-ray crystallographic studies and DFT calculations.