A dinuclear and a novel bi-cyclic hetero trinuclear bis(μ-tetrazolato) bridged nickel(II) Schiff base complexes [Ni₂(L¹)₂(PTZ)₂] (1) and [Ni₂(L²)₂(PTZ)₂Na(H₂O)]ClO₄·H₂O (2) {where HL¹ = 2-((2-(dimethylamino)ethylimino)methyl)-6-methoxyphenol, HL² = 2-((2-(methylamino)ethylimino)methyl)-6-methoxyphenol and HPTZ = 5-(2-pyridyl)tetrazole} have been synthesized by in situ 1,3-dipolar cyclo-addition and characterized by spectral analysis, X-ray crystallography, and variable-temperature magnetic susceptibility measurements. Both the complexes crystallize in monoclinic space group P2₁/c. Both the complexes feature double μ-NN′-tetrazolato bridged dinickel(II) structures, in which each nickel(II) is coordinated meridionally by a depronated terdentate Schiff base [(L¹)⁻ for 1 and (L²)⁻ for 2] and two nitrogen atoms of the (PTZ)⁻. A nitrogen atom from a symmetry related bridging (PTZ)⁻ coordinates to complete the distorted octahedral geometry of nickel(II). The phenoxo and methoxo oxygen atoms from two [NiL²] units and a water molecule coordinate to a sodium(I) to form the unique bi-cyclic trinuclear nickel(II)–sodium(I)–nickel(II) core in complex 2. Very strong π⋯π stacking is observed in complex 2 to form a supramolecular chain. The variable-temperature (1.8–300 K) magnetic susceptibility measurements show the presence of anti-ferromagnetic coupling between two nickel(II) centers for both complexes with J = −2.14(1) cm⁻¹ (for 1) and J = −1.20(2) cm⁻¹ (for 2). To obtain a better understanding of the magnetic exchange mechanism, quantum mechanical (DFT) calculations have been performed. The calculated J values [J_theo = −4.53 cm⁻¹ (for 1) and J_theo = −2.48 cm⁻¹ (for 2)] are in agreement with the values obtained experimentally.