Osmium(II) and osmium(III) complexes with a potentially hexadentate ligand [N,N,N′,N′‐tetrakis(2‐pyridylmethyl)ethylenediamine, (TPEN)] are synthesized. The X‐ray structural analysis of [OsII/III(TPEN)](PF6)2/3 complexes reveals that the chelating ligand exhibits a hexadentate coordination fashion and induces distortion from a regular octahedron to the osmium centers. Due to this distortion, the osmium(III) center becomes labile toward substitution by chloride and bromide anions. The osmium(III) complex, [OsIII(TPEN)](PF6)3, reacts with Cl– and Br– to give the anion bound six coordinated osmium(III) complex {[OsIII(Cl/Br)(κ5‐TPEN)]2+}, in which one of the equatorial pyridine donor groups is dissociated from the osmium(III) center. Kinetic and spectroscopic studies on the reaction with Br– at a low temperature suggest that the ligand substitution involves an associative mechanism via a seven coordinate osmium(III) complex intermediate involving Br–, [OsIIIBr(κ6‐TPEN)]2+. On the other hand, when the propylenediamine analogue [N,N,N′,N′‐tetrakis(2‐pyridylmethyl)propylenediamine, TPPN] is employed for the preparation of osmium(III) complex, [OsIII(TPPN)](PF6)3, the osmium(III) center displays a less distorted octahedral structure due to the longer alkylene tether. In contrast to the TPEN system, the osmium(III) complex of TPPN is reduced to the osmium(II) complex, [OsII(TPPN)](PF6)2, upon the reaction with Cl– and Br– yielding Cl2 or Br2. In this TPPN ligand system, associative mechanism involving an anion bound seven‐coordinate osmium(III) intermediate with the κ6‐hexadentate ligand is suggested to be operated as in the case of the TPEN ligand system.