The synthesis, crystal structures, and magnetic properties of linear pentanuclear complexes, [Ni(mu(5)-tpda)(4)X-2](n+) [tpdaH(2) = tripyridyldiamine, with different axial ligands X = Cl- (1), CN- (2), N-3(-) (3). and NCS- (4) (n = 0) and CH3CN (5) (n = 2)], are reported. All of the [Ni-5(tpda)(4)](2+) moieties are isostructral and involve a Ni-5 linear chain unit with all of the angle Ni-Ni-Ni being nearly 180 degrees, terminated by the two axial ligands. The pentanuclear linear metal chain is helically wrapped by four syn-syn-syn-syn type tpda(2-) ligands. There are two types of Ni-Ni distances existing in these complexes. The terminal Ni-Ni distances bonded with the axial ligand are longer (2.35-2.40 Angstrom) and affected by various axial ligands. The inner Ni-Ni distances are very short and remain constant (similar to 2.30 Angstrom). The relationship between terminal Ni-Ni distances and properties of the axial ligands on 1-5 will be discussed. Two terminal Ni(II) ions bonded with the axial ligands are in a square-pyramidal (NiN4X) environment and exhibit long Ni-N bonds (similar to 2.10 Angstrom) which are consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions display short Ni-N (similar to 1.90 Angstrom) bond distances which are consistent with a square-planar (NiN4), diamagnetic arrangement of a low-spin Ni(II) configuration. All compounds exhibit similar magnetic behavior, indicating an antiferromagnetic interaction of two terminal high-spin Ni(II) ions in these complexes. The XANES (X-ray absorption near-edge absorption spectroscopy) spectra also confirm the existence of the high-spin and low-spin Ni(II) ions in these complexes.