Abstract Thermal reactions of the complexes, [Ni(abi)4]X2·nH2O (abi, 2-aminobenzimidazole; X, Cl, Br, I, NO3, ClO4, or 1/2SO4; n, 3 for chloride and bromide, 1 for nitrate, and 0 for others) were investigated. Distinct color changes from orange to green were observed in the three complexes of chloride, bromide, and nitrate upon heating. It was found from the changes in absorption and far-IR spectra and magnetic susceptibilities under the thermal reactions that the complexes undergo transformation from square planar to tetragonally distorted octahedral structures, involving an increase in the coordination number from four to six. No such transformation could be found in the corresponding iodide, perchlorate, and sulfate. The effect of the introduced amino group on the structural conversion reactions of the abi complexes is discussed as compared with the corresponding benzimidazole complexes, [Ni(bimd)4]X2, which underwent transformation from square planar to octahedral structure upon heating when X is I or NO3.