In this work, it is found that 1,8-dihydroxyimino-1,2,7,8-tetraphenyl-3,6-diazocta-2,6-diene (^PhdoenH₂) could react with nickel(II) salt to yield a mononickel complex Ni(^Phdoen) (1) with unusual [2N2O]-coordinated mode, while the analogous diimine-dioxime ligands usually form the [4N]-coordinated mode. The novel complex 1 has been carefully characterized by ¹H NMR, elemental analysis, and X-ray diffraction structure analysis. The influences of the coordination modes on the structures and redox properties have been further investigated. Theoretical investigations revealed that the different coordination modes were ascribed to the thermodynamic properties of ligands.