The synthesis, crystal structures, and magnetic properties of linear pentanuclear complexes, [Ni(5)(&mgr;(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 CH(3)CN (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 angleNi-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 Å) and affected by various axial ligands. The inner Ni-Ni distances are very short and remain constant ( approximately 2.30 Å). 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 (NiN(4)X) environment and exhibit long Ni-N bonds ( approximately 2.10 Å) which are consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions display short Ni-N ( approximately 1.90 Å) bond distances which are consistent with a square-planar (NiN(4)), 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.