Three coordination polymers based on alkaline-earth metals (Ca, Sr, and Ba) have been synthesized with 3-amino-1,2,4-triazole-5-carboxylic acid (3-AmTrZAc). Structural analysis reveals that the Ca-based structure [Ca(3-AmTrZAc)(5-AmTrZAc)(H2O)] (1) is two-dimensional, and the Sr- and Ba-based structures [Sr(3-AmTrZAc)2(H2O)] (2) and [Ba(3-AmTrZAc)2(H2O)] (3) are three-dimensional. In the Ca-based structure 1, the 3-AmTrZAc reactant converts to 5-AmTrZAc. The relative stability of the two isomers is controlled by a dipole–ion interaction between the triazole ring and the carboxylate group. We have measured the band-gap energies, and the Ca-based structure has the largest band-gap energy. Density functional theoretical investigations showed that internal electric fields generated by the alkaline-earth counterions increase the band gap in the 5-AmTrZAc anion and lower it in the 3-AmTrZAc anion.