NiL.MX2 heterodinuclear complexes with a nuclear structure of NiII-MII (M = Zn, Cd, X = Cl, Br, I) were prepared in the dioxane medium by the use of an ONNO type ligand, Bis-N,N′(salicylidene)-1,3-propanediamine (LH2). The complexes prepared were characterized by the use of infrared (IR) spectroscopy, thermogravimetry (TG), semi-micro nitrogen, metal and halogen analysis. The molecular structures and unit cell properties of the compounds were elucidated by the X-ray diffraction (XRD) method. In parallel to the literature, MX2 group was observed to form μ‒ bridges with the phenolic oxygens of the organic ligand. The coordination sphere of NiL heterodinuclear complex in NiL group was observed to change depending upon the electronegativities of the atoms in MX2 group. The electronegativities of the atoms in the MX2 group affect the coordination sphere of NiL group of the heterodinuclear complex, and in parallel with the electronegativities of the atoms in MX2 group, it was observed that NiL unit coordinated the solvent molecules and the coordination sphere changed. In addition to this, the coordination bonds get smaller as the number of donor atoms nearby NiL decreased. Using Gaussian 09 software, the theoretical bond lengths and bond angles were calculated and compared with the experimental data. With Gaussian 09 software, it was determined how the occupancy levels of d orbitals of the metal atoms changed by coordination of Ni(II) ion. Also, the change of the differences between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the complexes was monitored. Display omitted •Ni(II) complex of ONNO type LH2 ligand has a square planar coordination sphere.•Zn(II) and Cd(II) halides are coordinated to NiL by μ-bridge over phenolic oxygen.•The electronegativity of atom in the halide affects the coordination sphere of NiL.•If ZnCl2 or ZnBr2 is attached, the coordination sphere is inevitable octahedral.•If CdI2 is attached, the coordination sphere remains as a square pyramidal.