Two binary nickel (II) supramolecular isomers, namely 1D-Ni and 2D-Ni with the same formula [Ni(2-cpa)2(H2O)]n (2-H2cpa = 2-carboxyphenoxyacetic acid), have been prepared by different synthetic methods and systematically characterized. Single-crystal X-ray diffraction analysis shows that the central NiII ion of 1D-Ni is located in a deformed square–pyramid bonded by one water and two 2-cpa anionic ligands. Each 2-cpa exhibits tetradentate bridging chelate-μ2 coordination mode and joins neighbouring NiII ions to form a 1-D zigzag chain structure. While for 2D-Ni, the NiII center is in a distorted octahedral geometry surrounded by one water and three 2-cpa. The 2-cpa displays pentadentate double bridging chelate-μ3 coordination pattern and links adjacent NiII ions to generate a corrugated (4,4)-connected layer. The magnetic measurements indicated that there are both weak ferromagnetic coupling between neighbouring NiII ions in 1D-Ni and 2D-Ni. In addition, the driving force for the formation of two isomers have been carefully analyzed and discussed.