The divalent transition metal complexes [Zn(L)2(H2O)2](Tere) (I), [Cd(L)2(H2O)2](Tere]) (II) and [Cd(L)2(HTere)2] (III) (L = 2,2’-biimidazole, Tere = terephthalate) have been synthesized under hydrothermal conditions and characterized by elemental analysis, IR spectrum, thermal analysis and single-crystal X-ray diffraction analysis. Complexes II and III have the same starting materials but possess different frame-works and are prepared from H2Biim and H2Tere under hydrothermal conditions with different pH values. The crystal structures show I and II have the same coordination circumstances and are coordinated by two H2O molecules and two neutral bidentate 2,2′-biimidazole ligands. The terephthalate acts as the counter anion. In contrast, complex III contains protonated carboxylate groups coordinated to the metal centre to give neutral species. Furthermore, based on the optimized structures, molecular frontier orbitals, Mulliken charges and IR spetra of complex I and III are investigated by density functional theory. Calculated results show that the energy gap (ΔEL-H) between HOMO and LUMO of complex III is bigger than that of I. It is revealed that complex III is more stable, and this calculated estimation corresponds with experimental analysis of TGA curves.