The synthesis, crystal structures, and magnetic properties of linear pentanuclear complexes, [Ni5(μ5-tpda)4X2]n+ [tpdaH2 = tripyridyldiamine, with different axial ligands X = Cl- (1), CN- (2), N3- (3), and NCS- (4) (n = 0) and CH3CN (5) (n = 2)], are reported. All of the [Ni5(tpda)4]2+ moieties are isostructral and involve a Ni5 linear chain unit with all of the ∠Ni−Ni−Ni being nearly 180°, terminated by the two axial ligands. The pentanuclear linear metal chain is helically wrapped by four syn−syn−syn−syn type tpda2- 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 A) and affected by various axial ligands. The inner Ni−Ni distances are very short and remain constant (∼2.30 A). 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 ...